collection.js

// @file collection.js - DBCollection support in the mongo shell
// db.colName is a DBCollection object
// or db["colName"]

if ((typeof DBCollection) == "undefined") {
    DBCollection = function(mongo, db, shortName, fullName) {
        this._mongo = mongo;
        this._db = db;
        this._shortName = shortName;
        this._fullName = fullName;

        this.verify();
    };
}

DBCollection.prototype.verify = function() {
    assert(this._fullName, "no fullName");
    assert(this._shortName, "no shortName");
    assert(this._db, "no db");

    assert.eq(this._fullName, this._db._name + "." + this._shortName, "name mismatch");

    assert(this._mongo, "no mongo in DBCollection");
    assert(this.getMongo(), "no mongo from getMongo()");
};

DBCollection.prototype.getName = function() {
    return this._shortName;
};

DBCollection.prototype.help = function() {
    var shortName = this.getName();
    print("DBCollection help");
    print("\tdb." + shortName + ".find().help() - show DBCursor help");
    print(
        "\tdb." + shortName +
        ".bulkWrite( operations, <optional params> ) - bulk execute write operations, optional parameters are: w, wtimeout, j");
    print(
        "\tdb." + shortName +
        ".count( query = {}, <optional params> ) - count the number of documents that matches the query, optional parameters are: limit, skip, hint, maxTimeMS");
    print(
        "\tdb." + shortName +
        ".copyTo(newColl) - duplicates collection by copying all documents to newColl; no indexes are copied.");
    print("\tdb." + shortName + ".convertToCapped(maxBytes) - calls {convertToCapped:'" +
          shortName + "', size:maxBytes}} command");
    print("\tdb." + shortName + ".createIndex(keypattern[,options])");
    print("\tdb." + shortName + ".createIndexes([keypatterns], <options>)");
    print("\tdb." + shortName + ".dataSize()");
    print(
        "\tdb." + shortName +
        ".deleteOne( filter, <optional params> ) - delete first matching document, optional parameters are: w, wtimeout, j");
    print(
        "\tdb." + shortName +
        ".deleteMany( filter, <optional params> ) - delete all matching documents, optional parameters are: w, wtimeout, j");
    print("\tdb." + shortName + ".distinct( key, query, <optional params> ) - e.g. db." +
          shortName + ".distinct( 'x' ), optional parameters are: maxTimeMS");
    print("\tdb." + shortName + ".drop() drop the collection");
    print("\tdb." + shortName + ".dropIndex(index) - e.g. db." + shortName +
          ".dropIndex( \"indexName\" ) or db." + shortName + ".dropIndex( { \"indexKey\" : 1 } )");
    print("\tdb." + shortName + ".dropIndexes()");
    print("\tdb." + shortName +
          ".ensureIndex(keypattern[,options]) - DEPRECATED, use createIndex() instead");
    print("\tdb." + shortName + ".explain().help() - show explain help");
    print("\tdb." + shortName + ".reIndex()");
    print(
        "\tdb." + shortName +
        ".find([query],[fields]) - query is an optional query filter. fields is optional set of fields to return.");
    print("\t                                              e.g. db." + shortName +
          ".find( {x:77} , {name:1, x:1} )");
    print("\tdb." + shortName + ".find(...).count()");
    print("\tdb." + shortName + ".find(...).limit(n)");
    print("\tdb." + shortName + ".find(...).skip(n)");
    print("\tdb." + shortName + ".find(...).sort(...)");
    print("\tdb." + shortName + ".findOne([query], [fields], [options], [readConcern])");
    print(
        "\tdb." + shortName +
        ".findOneAndDelete( filter, <optional params> ) - delete first matching document, optional parameters are: projection, sort, maxTimeMS");
    print(
        "\tdb." + shortName +
        ".findOneAndReplace( filter, replacement, <optional params> ) - replace first matching document, optional parameters are: projection, sort, maxTimeMS, upsert, returnNewDocument");
    print(
        "\tdb." + shortName +
        ".findOneAndUpdate( filter, update, <optional params> ) - update first matching document, optional parameters are: projection, sort, maxTimeMS, upsert, returnNewDocument");
    print("\tdb." + shortName + ".getDB() get DB object associated with collection");
    print("\tdb." + shortName + ".getPlanCache() get query plan cache associated with collection");
    print("\tdb." + shortName + ".getIndexes()");
    print("\tdb." + shortName + ".group( { key : ..., initial: ..., reduce : ...[, cond: ...] } )");
    print("\tdb." + shortName + ".insert(obj)");
    print(
        "\tdb." + shortName +
        ".insertOne( obj, <optional params> ) - insert a document, optional parameters are: w, wtimeout, j");
    print(
        "\tdb." + shortName +
        ".insertMany( [objects], <optional params> ) - insert multiple documents, optional parameters are: w, wtimeout, j");
    print("\tdb." + shortName + ".mapReduce( mapFunction , reduceFunction , <optional params> )");
    print(
        "\tdb." + shortName +
        ".aggregate( [pipeline], <optional params> ) - performs an aggregation on a collection; returns a cursor");
    print("\tdb." + shortName + ".remove(query)");
    print(
        "\tdb." + shortName +
        ".replaceOne( filter, replacement, <optional params> ) - replace the first matching document, optional parameters are: upsert, w, wtimeout, j");
    print("\tdb." + shortName +
          ".renameCollection( newName , <dropTarget> ) renames the collection.");
    print(
        "\tdb." + shortName +
        ".runCommand( name , <options> ) runs a db command with the given name where the first param is the collection name");
    print("\tdb." + shortName + ".save(obj)");
    print("\tdb." + shortName + ".stats({scale: N, indexDetails: true/false, " +
          "indexDetailsKey: <index key>, indexDetailsName: <index name>})");
    // print("\tdb." + shortName + ".diskStorageStats({[extent: <num>,] [granularity: <bytes>,]
    // ...}) - analyze record layout on disk");
    // print("\tdb." + shortName + ".pagesInRAM({[extent: <num>,] [granularity: <bytes>,] ...}) -
    // analyze resident memory pages");
    print("\tdb." + shortName +
          ".storageSize() - includes free space allocated to this collection");
    print("\tdb." + shortName + ".totalIndexSize() - size in bytes of all the indexes");
    print("\tdb." + shortName + ".totalSize() - storage allocated for all data and indexes");
    print(
        "\tdb." + shortName +
        ".update( query, object[, upsert_bool, multi_bool] ) - instead of two flags, you can pass an object with fields: upsert, multi");
    print(
        "\tdb." + shortName +
        ".updateOne( filter, update, <optional params> ) - update the first matching document, optional parameters are: upsert, w, wtimeout, j");
    print(
        "\tdb." + shortName +
        ".updateMany( filter, update, <optional params> ) - update all matching documents, optional parameters are: upsert, w, wtimeout, j");
    print("\tdb." + shortName + ".validate( <full> ) - SLOW");
    print("\tdb." + shortName + ".getShardVersion() - only for use with sharding");
    print("\tdb." + shortName +
          ".getShardDistribution() - prints statistics about data distribution in the cluster");
    print(
        "\tdb." + shortName +
        ".getSplitKeysForChunks( <maxChunkSize> ) - calculates split points over all chunks and returns splitter function");
    print(
        "\tdb." + shortName +
        ".getWriteConcern() - returns the write concern used for any operations on this collection, inherited from server/db if set");
    print(
        "\tdb." + shortName +
        ".setWriteConcern( <write concern doc> ) - sets the write concern for writes to the collection");
    print(
        "\tdb." + shortName +
        ".unsetWriteConcern( <write concern doc> ) - unsets the write concern for writes to the collection");
    print("\tdb." + shortName +
          ".latencyStats() - display operation latency histograms for this collection");
    // print("\tdb." + shortName + ".getDiskStorageStats({...}) - prints a summary of disk usage
    // statistics");
    // print("\tdb." + shortName + ".getPagesInRAM({...}) - prints a summary of storage pages
    // currently in physical memory");
    return __magicNoPrint;
};

DBCollection.prototype.getFullName = function() {
    return this._fullName;
};
DBCollection.prototype.getMongo = function() {
    return this._db.getMongo();
};
DBCollection.prototype.getDB = function() {
    return this._db;
};

DBCollection.prototype._makeCommand = function(cmd, params) {
    var c = {};
    c[cmd] = this.getName();
    if (params)
        Object.extend(c, params);
    return c;
};

DBCollection.prototype._dbCommand = function(cmd, params) {
    if (typeof(cmd) === "object")
        return this._db._dbCommand(cmd, {}, this.getQueryOptions());

    return this._db._dbCommand(this._makeCommand(cmd, params), {}, this.getQueryOptions());
};

// Like _dbCommand, but applies $readPreference
DBCollection.prototype._dbReadCommand = function(cmd, params) {
    if (typeof(cmd) === "object")
        return this._db._dbReadCommand(cmd, {}, this.getQueryOptions());

    return this._db._dbReadCommand(this._makeCommand(cmd, params), {}, this.getQueryOptions());
};

DBCollection.prototype.runCommand = DBCollection.prototype._dbCommand;

DBCollection.prototype.runReadCommand = DBCollection.prototype._dbReadCommand;

DBCollection.prototype._massageObject = function(q) {
    if (!q)
        return {};

    var type = typeof q;

    if (type == "function")
        return {$where: q};

    if (q.isObjectId)
        return {_id: q};

    if (type == "object")
        return q;

    if (type == "string") {
        // If the string is 24 hex characters, it is most likely an ObjectId.
        if (/^[0-9a-fA-F]{24}$/.test(q)) {
            return {_id: ObjectId(q)};
        }

        return {$where: q};
    }

    throw Error("don't know how to massage : " + type);

};

DBCollection.prototype._validateObject = function(o) {
    // Hidden property for testing purposes.
    if (this.getMongo()._skipValidation)
        return;

    if (typeof(o) != "object")
        throw Error("attempted to save a " + typeof(o) + " value.  document expected.");

    if (o._ensureSpecial && o._checkModify)
        throw Error("can't save a DBQuery object");
};

DBCollection._allowedFields = {
    $id: 1,
    $ref: 1,
    $db: 1
};

DBCollection.prototype._validateForStorage = function(o) {
    // Hidden property for testing purposes.
    if (this.getMongo()._skipValidation)
        return;

    this._validateObject(o);
    for (var k in o) {
        if (k.indexOf(".") >= 0) {
            throw Error("can't have . in field names [" + k + "]");
        }

        if (k.indexOf("$") == 0 && !DBCollection._allowedFields[k]) {
            throw Error("field names cannot start with $ [" + k + "]");
        }

        if (o[k] !== null && typeof(o[k]) === "object") {
            this._validateForStorage(o[k]);
        }
    }
};

DBCollection.prototype.find = function(query, fields, limit, skip, batchSize, options) {
    var cursor = new DBQuery(this._mongo,
                             this._db,
                             this,
                             this._fullName,
                             this._massageObject(query),
                             fields,
                             limit,
                             skip,
                             batchSize,
                             options || this.getQueryOptions());

    {
        const session = this.getDB().getSession();

        const readPreference = session._serverSession.client.getReadPreference(session);
        if (readPreference !== null) {
            cursor.readPref(readPreference.mode, readPreference.tags);
        }

        const readConcern = session._serverSession.client.getReadConcern(session);
        if (readConcern !== null) {
            cursor.readConcern(readConcern.level);
        }
    }

    return cursor;
};

DBCollection.prototype.findOne = function(query, fields, options, readConcern, collation) {
    var cursor = this.find(query, fields, -1 /* limit */, 0 /* skip*/, 0 /* batchSize */, options);

    if (readConcern) {
        cursor = cursor.readConcern(readConcern);
    }

    if (collation) {
        cursor = cursor.collation(collation);
    }

    if (!cursor.hasNext())
        return null;
    var ret = cursor.next();
    if (cursor.hasNext())
        throw Error("findOne has more than 1 result!");
    if (ret.$err)
        throw _getErrorWithCode(ret, "error " + tojson(ret));
    return ret;
};

DBCollection.prototype.insert = function(obj, options, _allow_dot) {
    if (!obj)
        throw Error("no object passed to insert!");

    var flags = 0;

    var wc = undefined;
    var allowDottedFields = false;
    if (options === undefined) {
        // do nothing
    } else if (typeof(options) == 'object') {
        if (options.ordered === undefined) {
            // do nothing, like above
        } else {
            flags = options.ordered ? 0 : 1;
        }

        if (options.writeConcern)
            wc = options.writeConcern;
        if (options.allowdotted)
            allowDottedFields = true;
    } else {
        flags = options;
    }

    // 1 = continueOnError, which is synonymous with unordered in the write commands/bulk-api
    var ordered = ((flags & 1) == 0);

    if (!wc)
        wc = this.getWriteConcern();

    var result = undefined;
    var startTime =
        (typeof(_verboseShell) === 'undefined' || !_verboseShell) ? 0 : new Date().getTime();

    if (this.getMongo().writeMode() != "legacy") {
        // Bit 1 of option flag is continueOnError. Bit 0 (stop on error) is the default.
        var bulk = ordered ? this.initializeOrderedBulkOp() : this.initializeUnorderedBulkOp();
        var isMultiInsert = Array.isArray(obj);

        if (isMultiInsert) {
            obj.forEach(function(doc) {
                bulk.insert(doc);
            });
        } else {
            bulk.insert(obj);
        }

        try {
            result = bulk.execute(wc);
            if (!isMultiInsert)
                result = result.toSingleResult();
        } catch (ex) {
            if (ex instanceof BulkWriteError) {
                result = isMultiInsert ? ex.toResult() : ex.toSingleResult();
            } else if (ex instanceof WriteCommandError) {
                result = isMultiInsert ? ex : ex.toSingleResult();
            } else {
                // Other exceptions rethrown as-is.
                throw ex;
            }
        }
    } else {
        if (!_allow_dot) {
            this._validateForStorage(obj);
        }

        if (typeof(obj._id) == "undefined" && !Array.isArray(obj)) {
            var tmp = obj;  // don't want to modify input
            obj = {_id: new ObjectId()};
            for (var key in tmp) {
                obj[key] = tmp[key];
            }
        }

        this.getMongo().insert(this._fullName, obj, flags);

        // enforce write concern, if required
        if (wc)
            result = this.runCommand("getLastError", wc instanceof WriteConcern ? wc.toJSON() : wc);
    }

    this._lastID = obj._id;
    this._printExtraInfo("Inserted", startTime);
    return result;
};

DBCollection.prototype._validateRemoveDoc = function(doc) {
    // Hidden property for testing purposes.
    if (this.getMongo()._skipValidation)
        return;

    for (var k in doc) {
        if (k == "_id" && typeof(doc[k]) == "undefined") {
            throw new Error("can't have _id set to undefined in a remove expression");
        }
    }
};

/**
 * Does validation of the remove args. Throws if the parse is not successful, otherwise
 * returns a document {query: <query>, justOne: <limit>, wc: <writeConcern>}.
 */
DBCollection.prototype._parseRemove = function(t, justOne) {
    if (undefined === t)
        throw Error("remove needs a query");

    var query = this._massageObject(t);

    var wc = undefined;
    var collation = undefined;
    if (typeof(justOne) === "object") {
        var opts = justOne;
        wc = opts.writeConcern;
        justOne = opts.justOne;
        collation = opts.collation;
    }

    // Normalize "justOne" to a bool.
    justOne = justOne ? true : false;

    // Handle write concern.
    if (!wc) {
        wc = this.getWriteConcern();
    }

    return {"query": query, "justOne": justOne, "wc": wc, "collation": collation};
};

DBCollection.prototype.remove = function(t, justOne) {
    var parsed = this._parseRemove(t, justOne);
    var query = parsed.query;
    var justOne = parsed.justOne;
    var wc = parsed.wc;
    var collation = parsed.collation;

    var result = undefined;
    var startTime =
        (typeof(_verboseShell) === 'undefined' || !_verboseShell) ? 0 : new Date().getTime();

    if (this.getMongo().writeMode() != "legacy") {
        var bulk = this.initializeOrderedBulkOp();
        var removeOp = bulk.find(query);

        if (collation) {
            removeOp.collation(collation);
        }

        if (justOne) {
            removeOp.removeOne();
        } else {
            removeOp.remove();
        }

        try {
            result = bulk.execute(wc).toSingleResult();
        } catch (ex) {
            if (ex instanceof BulkWriteError || ex instanceof WriteCommandError) {
                result = ex.toSingleResult();
            } else {
                // Other exceptions thrown
                throw Error(ex);
            }
        }
    } else {
        if (collation) {
            throw new Error("collation requires use of write commands");
        }

        this._validateRemoveDoc(t);
        this.getMongo().remove(this._fullName, query, justOne);

        // enforce write concern, if required
        if (wc)
            result = this.runCommand("getLastError", wc instanceof WriteConcern ? wc.toJSON() : wc);
    }

    this._printExtraInfo("Removed", startTime);
    return result;
};

DBCollection.prototype._validateUpdateDoc = function(doc) {
    // Hidden property for testing purposes.
    if (this.getMongo()._skipValidation)
        return;

    var firstKey = null;
    for (var key in doc) {
        firstKey = key;
        break;
    }

    if (firstKey != null && firstKey[0] == '$') {
        // for mods we only validate partially, for example keys may have dots
        this._validateObject(doc);
    } else {
        // we're basically inserting a brand new object, do full validation
        this._validateForStorage(doc);
    }
};

/**
 * Does validation of the update args. Throws if the parse is not successful, otherwise
 * returns a document containing fields for query, obj, upsert, multi, wc, collation, and
 * arrayFilters.
 *
 * Throws if the arguments are invalid.
 */
DBCollection.prototype._parseUpdate = function(query, obj, upsert, multi) {
    if (!query)
        throw Error("need a query");
    if (!obj)
        throw Error("need an object");

    var wc = undefined;
    var collation = undefined;
    var arrayFilters = undefined;
    // can pass options via object for improved readability
    if (typeof(upsert) === "object") {
        if (multi) {
            throw Error("Fourth argument must be empty when specifying " +
                        "upsert and multi with an object.");
        }

        var opts = upsert;
        multi = opts.multi;
        wc = opts.writeConcern;
        upsert = opts.upsert;
        collation = opts.collation;
        arrayFilters = opts.arrayFilters;
    }

    // Normalize 'upsert' and 'multi' to booleans.
    upsert = upsert ? true : false;
    multi = multi ? true : false;

    if (!wc) {
        wc = this.getWriteConcern();
    }

    return {
        "query": query,
        "obj": obj,
        "upsert": upsert,
        "multi": multi,
        "wc": wc,
        "collation": collation,
        "arrayFilters": arrayFilters
    };
};

DBCollection.prototype.update = function(query, obj, upsert, multi) {
    var parsed = this._parseUpdate(query, obj, upsert, multi);
    var query = parsed.query;
    var obj = parsed.obj;
    var upsert = parsed.upsert;
    var multi = parsed.multi;
    var wc = parsed.wc;
    var collation = parsed.collation;
    var arrayFilters = parsed.arrayFilters;

    var result = undefined;
    var startTime =
        (typeof(_verboseShell) === 'undefined' || !_verboseShell) ? 0 : new Date().getTime();

    if (this.getMongo().writeMode() != "legacy") {
        var bulk = this.initializeOrderedBulkOp();
        var updateOp = bulk.find(query);

        if (upsert) {
            updateOp = updateOp.upsert();
        }

        if (collation) {
            updateOp.collation(collation);
        }

        if (arrayFilters) {
            updateOp.arrayFilters(arrayFilters);
        }

        if (multi) {
            updateOp.update(obj);
        } else {
            updateOp.updateOne(obj);
        }

        try {
            result = bulk.execute(wc).toSingleResult();
        } catch (ex) {
            if (ex instanceof BulkWriteError || ex instanceof WriteCommandError) {
                result = ex.toSingleResult();
            } else {
                // Other exceptions thrown
                throw Error(ex);
            }
        }
    } else {
        if (collation) {
            throw new Error("collation requires use of write commands");
        }

        if (arrayFilters) {
            throw new Error("arrayFilters requires use of write commands");
        }

        this._validateUpdateDoc(obj);
        this.getMongo().update(this._fullName, query, obj, upsert, multi);

        // Enforce write concern, if required
        if (wc) {
            result = this.runCommand("getLastError", wc instanceof WriteConcern ? wc.toJSON() : wc);
        }
    }

    this._printExtraInfo("Updated", startTime);
    return result;
};

DBCollection.prototype.save = function(obj, opts) {
    if (obj == null)
        throw Error("can't save a null");

    if (typeof(obj) == "number" || typeof(obj) == "string")
        throw Error("can't save a number or string");

    if (typeof(obj._id) == "undefined") {
        obj._id = new ObjectId();
        return this.insert(obj, opts);
    } else {
        return this.update({_id: obj._id}, obj, Object.merge({upsert: true}, opts));
    }
};

DBCollection.prototype._genIndexName = function(keys) {
    var name = "";
    for (var k in keys) {
        var v = keys[k];
        if (typeof v == "function")
            continue;

        if (name.length > 0)
            name += "_";
        name += k + "_";

        name += v;
    }
    return name;
};

DBCollection.prototype._indexSpec = function(keys, options) {
    var ret = {ns: this._fullName, key: keys, name: this._genIndexName(keys)};

    if (!options) {
    } else if (typeof(options) == "string")
        ret.name = options;
    else if (typeof(options) == "boolean")
        ret.unique = true;
    else if (typeof(options) == "object") {
        if (Array.isArray(options)) {
            if (options.length > 3) {
                throw new Error("Index options that are supplied in array form may only specify" +
                                " three values: name, unique, dropDups");
            }
            var nb = 0;
            for (var i = 0; i < options.length; i++) {
                if (typeof(options[i]) == "string")
                    ret.name = options[i];
                else if (typeof(options[i]) == "boolean") {
                    if (options[i]) {
                        if (nb == 0)
                            ret.unique = true;
                        if (nb == 1)
                            ret.dropDups = true;
                    }
                    nb++;
                }
            }
        } else {
            Object.extend(ret, options);
        }
    } else {
        throw Error("can't handle: " + typeof(options));
    }

    return ret;
};

DBCollection.prototype.createIndex = function(keys, options) {
    return this.createIndexes([keys], options);
};

DBCollection.prototype.createIndexes = function(keys, options) {
    if (!Array.isArray(keys)) {
        throw new Error("createIndexes first argument should be an array");
    }

    var indexSpecs = Array(keys.length);
    for (var i = 0; i < indexSpecs.length; i++) {
        indexSpecs[i] = this._indexSpec(keys[i], options);
    }

    if (this.getMongo().writeMode() == "commands") {
        for (var i = 0; i < indexSpecs.length; i++) {
            delete (indexSpecs[i].ns);  // ns is passed to the first element in the command.
        }
        return this._db.runCommand({createIndexes: this.getName(), indexes: indexSpecs});
    } else if (this.getMongo().writeMode() == "compatibility") {
        // Use the downconversion machinery of the bulk api to do a safe write, report response as a
        // command response
        var result = this._db.getCollection("system.indexes").insert(indexSpecs, 0, true);

        if (result.hasWriteErrors() || result.hasWriteConcernError()) {
            // Return the first error
            var error = result.hasWriteErrors() ? result.getWriteErrors()[0]
                                                : result.getWriteConcernError();
            return {ok: 0.0, code: error.code, errmsg: error.errmsg};
        } else {
            return {ok: 1.0};
        }
    } else {
        this._db.getCollection("system.indexes").insert(indexSpecs, 0, true);
    }
};

DBCollection.prototype.ensureIndex = function(keys, options) {
    var result = this.createIndex(keys, options);

    if (this.getMongo().writeMode() != "legacy") {
        return result;
    }

    err = this.getDB().getLastErrorObj();
    if (err.err) {
        return err;
    }
    // nothing returned on success
};

DBCollection.prototype.reIndex = function() {
    return this._db.runCommand({reIndex: this.getName()});
};

DBCollection.prototype.dropIndexes = function() {
    if (arguments.length)
        throw Error("dropIndexes doesn't take arguments");

    var res = this._db.runCommand({deleteIndexes: this.getName(), index: "*"});
    assert(res, "no result from dropIndex result");
    if (res.ok)
        return res;

    if (res.errmsg.match(/not found/))
        return res;

    throw _getErrorWithCode(res, "error dropping indexes : " + tojson(res));
};

DBCollection.prototype.drop = function() {
    if (arguments.length > 0)
        throw Error("drop takes no argument");
    var ret = this._db.runCommand({drop: this.getName()});
    if (!ret.ok) {
        if (ret.errmsg == "ns not found")
            return false;
        throw _getErrorWithCode(ret, "drop failed: " + tojson(ret));
    }
    return true;
};

DBCollection.prototype.findAndModify = function(args) {
    var cmd = {findandmodify: this.getName()};
    for (var key in args) {
        cmd[key] = args[key];
    }

    {
        const kWireVersionSupportingRetryableWrites = 6;
        const serverSupportsRetryableWrites =
            this.getMongo().getMinWireVersion() <= kWireVersionSupportingRetryableWrites &&
            kWireVersionSupportingRetryableWrites <= this.getMongo().getMaxWireVersion();

        const session = this.getDB().getSession();
        if (serverSupportsRetryableWrites && session.getOptions().shouldRetryWrites() &&
            session._serverSession.canRetryWrites(cmd)) {
            cmd = session._serverSession.assignTransactionNumber(cmd);
        }
    }

    var ret = this._db.runCommand(cmd);
    if (!ret.ok) {
        if (ret.errmsg == "No matching object found") {
            return null;
        }
        throw _getErrorWithCode(ret, "findAndModifyFailed failed: " + tojson(ret));
    }
    return ret.value;
};

DBCollection.prototype.renameCollection = function(newName, dropTarget) {
    if (arguments.length === 1 && typeof newName === 'object') {
        if (newName.hasOwnProperty('dropTarget')) {
            dropTarget = newName['dropTarget'];
        }
        newName = newName['to'];
    }
    if (typeof dropTarget === 'undefined') {
        dropTarget = false;
    }
    if (typeof newName !== 'string' || typeof dropTarget !== 'boolean') {
        throw Error(
            'renameCollection must either take a string and an optional boolean or an object.');
    }
    return this._db._adminCommand({
        renameCollection: this._fullName,
        to: this._db._name + "." + newName,
        dropTarget: dropTarget
    });
};

// Display verbose information about the operation
DBCollection.prototype._printExtraInfo = function(action, startTime) {
    if (typeof _verboseShell === 'undefined' || !_verboseShell) {
        __callLastError = true;
        return;
    }

    // explicit w:1 so that replset getLastErrorDefaults aren't used here which would be bad.
    var res = this._db.getLastErrorCmd(1);
    if (res) {
        if (res.err != undefined && res.err != null) {
            // error occurred, display it
            print(res.err);
            return;
        }

        var info = action + " ";
        // hack for inserted because res.n is 0
        info += action != "Inserted" ? res.n : 1;
        if (res.n > 0 && res.updatedExisting != undefined)
            info += " " + (res.updatedExisting ? "existing" : "new");
        info += " record(s)";
        var time = new Date().getTime() - startTime;
        info += " in " + time + "ms";
        print(info);
    }
};

DBCollection.prototype.validate = function(full) {
    var cmd = {validate: this.getName()};

    if (typeof(full) == 'object')  // support arbitrary options here
        Object.extend(cmd, full);
    else
        cmd.full = full;

    var res = this._db.runCommand(cmd);

    if (typeof(res.valid) == 'undefined') {
        // old-style format just put everything in a string. Now using proper fields

        res.valid = false;

        var raw = res.result || res.raw;

        if (raw) {
            var str = "-" + tojson(raw);
            res.valid = !(str.match(/exception/) || str.match(/corrupt/));

            var p = /lastExtentSize:(\d+)/;
            var r = p.exec(str);
            if (r) {
                res.lastExtentSize = Number(r[1]);
            }
        }
    }

    return res;
};

/**
 * Invokes the storageDetails command to provide aggregate and (if requested) detailed information
 * regarding the layout of records and deleted records in the collection extents.
 * getDiskStorageStats provides a human-readable summary of the command output
 */
DBCollection.prototype.diskStorageStats = function(opt) {
    var cmd = {storageDetails: this.getName(), analyze: 'diskStorage'};
    if (typeof(opt) == 'object')
        Object.extend(cmd, opt);

    var res = this._db.runCommand(cmd);
    if (!res.ok && res.errmsg.match(/no such cmd/)) {
        print("this command requires starting mongod with --enableExperimentalStorageDetailsCmd");
    }
    return res;
};

// Refer to diskStorageStats
DBCollection.prototype.getDiskStorageStats = function(params) {
    var stats = this.diskStorageStats(params);
    if (!stats.ok) {
        print("error executing storageDetails command: " + stats.errmsg);
        return;
    }

    print("\n    " + "size".pad(9) + " " + "# recs".pad(10) + " " +
          "[===occupied by BSON=== ---occupied by padding---       free           ]" + "  " +
          "bson".pad(8) + " " + "rec".pad(8) + " " + "padding".pad(8));
    print();

    var BAR_WIDTH = 70;

    var formatSliceData = function(data) {
        var bar = _barFormat(
            [
              [data.bsonBytes / data.onDiskBytes, "="],
              [(data.recBytes - data.bsonBytes) / data.onDiskBytes, "-"]
            ],
            BAR_WIDTH);

        return sh._dataFormat(data.onDiskBytes).pad(9) + " " + data.numEntries.toFixed(0).pad(10) +
            " " + bar + "  " + (data.bsonBytes / data.onDiskBytes).toPercentStr().pad(8) + " " +
            (data.recBytes / data.onDiskBytes).toPercentStr().pad(8) + " " +
            (data.recBytes / data.bsonBytes).toFixed(4).pad(8);
    };

    var printExtent = function(ex, rng) {
        print("--- extent " + rng + " ---");
        print("tot " + formatSliceData(ex));
        print();
        if (ex.slices) {
            for (var c = 0; c < ex.slices.length; c++) {
                var slice = ex.slices[c];
                print(("" + c).pad(3) + " " + formatSliceData(slice));
            }
            print();
        }
    };

    if (stats.extents) {
        print("--- extent overview ---\n");
        for (var i = 0; i < stats.extents.length; i++) {
            var ex = stats.extents[i];
            print(("" + i).pad(3) + " " + formatSliceData(ex));
        }
        print();
        if (params && (params.granularity || params.numberOfSlices)) {
            for (var i = 0; i < stats.extents.length; i++) {
                printExtent(stats.extents[i], i);
            }
        }
    } else {
        printExtent(stats, "range " + stats.range);
    }

};

/**
 * Invokes the storageDetails command to report the percentage of virtual memory pages of the
 * collection storage currently in physical memory (RAM).
 * getPagesInRAM provides a human-readable summary of the command output
 */
DBCollection.prototype.pagesInRAM = function(opt) {
    var cmd = {storageDetails: this.getName(), analyze: 'pagesInRAM'};
    if (typeof(opt) == 'object')
        Object.extend(cmd, opt);

    var res = this._db.runCommand(cmd);
    if (!res.ok && res.errmsg.match(/no such cmd/)) {
        print("this command requires starting mongod with --enableExperimentalStorageDetailsCmd");
    }
    return res;
};

// Refer to pagesInRAM
DBCollection.prototype.getPagesInRAM = function(params) {
    var stats = this.pagesInRAM(params);
    if (!stats.ok) {
        print("error executing storageDetails command: " + stats.errmsg);
        return;
    }

    var BAR_WIDTH = 70;
    var formatExtentData = function(data) {
        return "size".pad(8) + " " + _barFormat([[data.inMem, '=']], BAR_WIDTH) + "  " +
            data.inMem.toPercentStr().pad(7);
    };

    var printExtent = function(ex, rng) {
        print("--- extent " + rng + " ---");
        print("tot " + formatExtentData(ex));
        print();
        if (ex.slices) {
            print("\tslices, percentage of pages in memory (< .1% : ' ', <25% : '.', " +
                  "<50% : '_', <75% : '=', >75% : '#')");
            print();
            print("\t" + "offset".pad(8) + "  [slices...] (each slice is " +
                  sh._dataFormat(ex.sliceBytes) + ")");
            line = "\t" + ("" + 0).pad(8) + "  [";
            for (var c = 0; c < ex.slices.length; c++) {
                if (c % 80 == 0 && c != 0) {
                    print(line + "]");
                    line = "\t" + sh._dataFormat(ex.sliceBytes * c).pad(8) + "  [";
                }
                var inMem = ex.slices[c];
                if (inMem <= .001)
                    line += " ";
                else if (inMem <= .25)
                    line += ".";
                else if (inMem <= .5)
                    line += "_";
                else if (inMem <= .75)
                    line += "=";
                else
                    line += "#";
            }
            print(line + "]");
            print();
        }
    };

    if (stats.extents) {
        print("--- extent overview ---\n");
        for (var i = 0; i < stats.extents.length; i++) {
            var ex = stats.extents[i];
            print(("" + i).pad(3) + " " + formatExtentData(ex));
        }
        print();
        if (params && (params.granularity || params.numberOfSlices)) {
            for (var i = 0; i < stats.extents.length; i++) {
                printExtent(stats.extents[i], i);
            }
        } else {
            print("use getPagesInRAM({granularity: _bytes_}) or " +
                  "getPagesInRAM({numberOfSlices: _num_} for details");
            print("use pagesInRAM(...) for json output, same parameters apply");
        }
    } else {
        printExtent(stats, "range " + stats.range);
    }
};

DBCollection.prototype.getShardVersion = function() {
    return this._db._adminCommand({getShardVersion: this._fullName});
};

DBCollection.prototype._getIndexesSystemIndexes = function(filter) {
    var si = this.getDB().getCollection("system.indexes");
    var query = {ns: this.getFullName()};
    if (filter)
        query = Object.extend(query, filter);
    return si.find(query).toArray();
};

DBCollection.prototype._getIndexesCommand = function(filter) {
    var res = this.runCommand("listIndexes", filter);

    if (!res.ok) {
        if (res.code == 59) {
            // command doesn't exist, old mongod
            return null;
        }

        if (res.code == 26) {
            // NamespaceNotFound, for compatability, return []
            return [];
        }

        if (res.errmsg && res.errmsg.startsWith("no such cmd")) {
            return null;
        }

        throw _getErrorWithCode(res, "listIndexes failed: " + tojson(res));
    }

    return new DBCommandCursor(res._mongo, res).toArray();
};

DBCollection.prototype.getIndexes = function(filter) {
    var res = this._getIndexesCommand(filter);
    if (res) {
        return res;
    }
    return this._getIndexesSystemIndexes(filter);
};

DBCollection.prototype.getIndices = DBCollection.prototype.getIndexes;
DBCollection.prototype.getIndexSpecs = DBCollection.prototype.getIndexes;

DBCollection.prototype.getIndexKeys = function() {
    return this.getIndexes().map(function(i) {
        return i.key;
    });
};

DBCollection.prototype.hashAllDocs = function() {
    var cmd = {dbhash: 1, collections: [this._shortName]};
    var res = this._dbCommand(cmd);
    var hash = res.collections[this._shortName];
    assert(hash);
    assert(typeof(hash) == "string");
    return hash;
};

/**
 * <p>Drop a specified index.</p>
 *
 * <p>
 * "index" is the name of the index in the system.indexes name field (run db.system.indexes.find()
 *to
 *  see example data), or an object holding the key(s) used to create the index.
 * For example:
 *  db.collectionName.dropIndex( "myIndexName" );
 *  db.collectionName.dropIndex( { "indexKey" : 1 } );
 * </p>
 *
 * @param {String} name or key object of index to delete.
 * @return A result object.  result.ok will be true if successful.
 */
DBCollection.prototype.dropIndex = function(index) {
    assert(index, "need to specify index to dropIndex");
    var res = this._dbCommand("deleteIndexes", {index: index});
    return res;
};

DBCollection.prototype.copyTo = function(newName) {
    return this.getDB().eval(function(collName, newName) {
        var from = db[collName];
        var to = db[newName];
        to.ensureIndex({_id: 1});
        var count = 0;

        var cursor = from.find();
        while (cursor.hasNext()) {
            var o = cursor.next();
            count++;
            to.save(o);
        }

        return count;
    }, this.getName(), newName);
};

DBCollection.prototype.getCollection = function(subName) {
    return this._db.getCollection(this._shortName + "." + subName);
};

/**
  * scale: The scale at which to deliver results. Unless specified, this command returns all data
  *        in bytes.
  * indexDetails: Includes indexDetails field in results. Default: false.
  * indexDetailsKey: If indexDetails is true, filter contents in indexDetails by this index key.
  * indexDetailsname: If indexDetails is true, filter contents in indexDetails by this index name.
  *
  * It is an error to provide both indexDetailsKey and indexDetailsName.
  */
DBCollection.prototype.stats = function(args) {
    'use strict';

    // For backwards compatibility with db.collection.stats(scale).
    var scale = isObject(args) ? args.scale : args;

    var options = isObject(args) ? args : {};
    if (options.indexDetailsKey && options.indexDetailsName) {
        throw new Error('Cannot filter indexDetails on both indexDetailsKey and ' +
                        'indexDetailsName');
    }
    // collStats can run on a secondary, so we need to apply readPreference
    var res = this._db.runReadCommand({collStats: this._shortName, scale: scale});
    if (!res.ok) {
        return res;
    }

    var getIndexName = function(collection, indexKey) {
        if (!isObject(indexKey))
            return undefined;
        var indexName;
        collection.getIndexes().forEach(function(spec) {
            if (friendlyEqual(spec.key, options.indexDetailsKey)) {
                indexName = spec.name;
            }
        });
        return indexName;
    };

    var filterIndexName = options.indexDetailsName || getIndexName(this, options.indexDetailsKey);

    var updateStats = function(stats, keepIndexDetails, indexName) {
        if (!stats.indexDetails)
            return;
        if (!keepIndexDetails) {
            delete stats.indexDetails;
            return;
        }
        if (!indexName)
            return;
        for (var key in stats.indexDetails) {
            if (key == indexName)
                continue;
            delete stats.indexDetails[key];
        }
    };

    updateStats(res, options.indexDetails, filterIndexName);

    if (res.sharded) {
        for (var shardName in res.shards) {
            updateStats(res.shards[shardName], options.indexDetails, filterIndexName);
        }
    }

    return res;
};

DBCollection.prototype.dataSize = function() {
    return this.stats().size;
};

DBCollection.prototype.storageSize = function() {
    return this.stats().storageSize;
};

DBCollection.prototype.totalIndexSize = function(verbose) {
    var stats = this.stats();
    if (verbose) {
        for (var ns in stats.indexSizes) {
            print(ns + "\t" + stats.indexSizes[ns]);
        }
    }
    return stats.totalIndexSize;
};

DBCollection.prototype.totalSize = function() {
    var total = this.storageSize();
    var totalIndexSize = this.totalIndexSize();
    if (totalIndexSize) {
        total += totalIndexSize;
    }
    return total;
};

DBCollection.prototype.convertToCapped = function(bytes) {
    if (!bytes)
        throw Error("have to specify # of bytes");
    return this._dbCommand({convertToCapped: this._shortName, size: bytes});
};

DBCollection.prototype.exists = function() {
    var res = this._db.runCommand("listCollections", {filter: {name: this._shortName}});
    if (res.ok) {
        var cursor = new DBCommandCursor(res._mongo, res);
        if (!cursor.hasNext())
            return null;
        return cursor.next();
    }

    if (res.errmsg && res.errmsg.startsWith("no such cmd")) {
        return this._db.system.namespaces.findOne({name: this._fullName});
    }

    throw _getErrorWithCode(res, "listCollections failed: " + tojson(res));
};

DBCollection.prototype.isCapped = function() {
    var e = this.exists();
    return (e && e.options && e.options.capped) ? true : false;
};

//
// CRUD specification aggregation cursor extension
//
DBCollection.prototype.aggregate = function(pipeline, aggregateOptions) {
    if (!(pipeline instanceof Array)) {
        // Support legacy varargs form. Also handles db.foo.aggregate().
        pipeline = Array.from(arguments);
        aggregateOptions = {};
    } else if (aggregateOptions === undefined) {
        aggregateOptions = {};
    }

    const cmdObj = this._makeCommand("aggregate", {pipeline: pipeline});

    return this._db._runAggregate(cmdObj, aggregateOptions);
};

DBCollection.prototype.group = function(params) {
    params.ns = this._shortName;
    return this._db.group(params);
};

DBCollection.prototype.groupcmd = function(params) {
    params.ns = this._shortName;
    return this._db.groupcmd(params);
};

MapReduceResult = function(db, o) {
    Object.extend(this, o);
    this._o = o;
    this._keys = Object.keySet(o);
    this._db = db;
    if (this.result != null) {
        this._coll = this._db.getCollection(this.result);
    }
};

MapReduceResult.prototype._simpleKeys = function() {
    return this._o;
};

MapReduceResult.prototype.find = function() {
    if (this.results)
        return this.results;
    return DBCollection.prototype.find.apply(this._coll, arguments);
};

MapReduceResult.prototype.drop = function() {
    if (this._coll) {
        return this._coll.drop();
    }
};

/**
* just for debugging really
*/
MapReduceResult.prototype.convertToSingleObject = function() {
    var z = {};
    var it = this.results != null ? this.results : this._coll.find();
    it.forEach(function(a) {
        z[a._id] = a.value;
    });
    return z;
};

DBCollection.prototype.convertToSingleObject = function(valueField) {
    var z = {};
    this.find().forEach(function(a) {
        z[a._id] = a[valueField];
    });
    return z;
};

/**
* @param optional object of optional fields;
*/
DBCollection.prototype.mapReduce = function(map, reduce, optionsOrOutString) {
    var c = {mapreduce: this._shortName, map: map, reduce: reduce};
    assert(optionsOrOutString, "need to supply an optionsOrOutString");

    if (typeof(optionsOrOutString) == "string")
        c["out"] = optionsOrOutString;
    else
        Object.extend(c, optionsOrOutString);

    var raw;

    if (c["out"].hasOwnProperty("inline") && c["out"]["inline"] === 1) {
        // if inline output is specified, we need to apply readPreference on the command
        // as it could be run on a secondary
        raw = this._db.runReadCommand(c);
    } else {
        raw = this._db.runCommand(c);
    }

    if (!raw.ok) {
        __mrerror__ = raw;
        throw _getErrorWithCode(raw, "map reduce failed:" + tojson(raw));
    }
    return new MapReduceResult(this._db, raw);

};

DBCollection.prototype.toString = function() {
    return this.getFullName();
};

DBCollection.prototype.toString = function() {
    return this.getFullName();
};

DBCollection.prototype.tojson = DBCollection.prototype.toString;

DBCollection.prototype.shellPrint = DBCollection.prototype.toString;

DBCollection.autocomplete = function(obj) {
    var colls = DB.autocomplete(obj.getDB());
    var ret = [];
    for (var i = 0; i < colls.length; i++) {
        var c = colls[i];
        if (c.length <= obj.getName().length)
            continue;
        if (c.slice(0, obj.getName().length + 1) != obj.getName() + '.')
            continue;

        ret.push(c.slice(obj.getName().length + 1));
    }
    return ret;
};

// Sharding additions

/*
Usage :

mongo <mongos>
> load('path-to-file/shardingAdditions.js')
Loading custom sharding extensions...
true

> var collection = db.getMongo().getCollection("foo.bar")
> collection.getShardDistribution() // prints statistics related to the collection's data
distribution

> collection.getSplitKeysForChunks() // generates split points for all chunks in the collection,
based on the
                                     // default maxChunkSize or alternately a specified chunk size
> collection.getSplitKeysForChunks( 10 ) // Mb

> var splitter = collection.getSplitKeysForChunks() // by default, the chunks are not split, the
keys are just
                                                    // found.  A splitter function is returned which
will actually
                                                    // do the splits.

> splitter() // ! Actually executes the splits on the cluster !

*/

DBCollection.prototype.getShardDistribution = function() {

    var stats = this.stats();

    if (!stats.sharded) {
        print("Collection " + this + " is not sharded.");
        return;
    }

    var config = this.getDB().getSiblingDB("config");

    var numChunks = 0;

    for (var shard in stats.shards) {
        var shardDoc = config.shards.findOne({_id: shard});

        print("\nShard " + shard + " at " + shardDoc.host);

        var shardStats = stats.shards[shard];

        var chunks = config.chunks.find({_id: sh._collRE(this), shard: shard}).toArray();

        numChunks += chunks.length;

        var estChunkData = shardStats.size / chunks.length;
        var estChunkCount = Math.floor(shardStats.count / chunks.length);

        print(" data : " + sh._dataFormat(shardStats.size) + " docs : " + shardStats.count +
              " chunks : " + chunks.length);
        print(" estimated data per chunk : " + sh._dataFormat(estChunkData));
        print(" estimated docs per chunk : " + estChunkCount);
    }

    print("\nTotals");
    print(" data : " + sh._dataFormat(stats.size) + " docs : " + stats.count + " chunks : " +
          numChunks);
    for (var shard in stats.shards) {
        var shardStats = stats.shards[shard];

        var estDataPercent = Math.floor(shardStats.size / stats.size * 10000) / 100;
        var estDocPercent = Math.floor(shardStats.count / stats.count * 10000) / 100;

        print(" Shard " + shard + " contains " + estDataPercent + "% data, " + estDocPercent +
              "% docs in cluster, " + "avg obj size on shard : " +
              sh._dataFormat(stats.shards[shard].avgObjSize));
    }

    print("\n");

};

DBCollection.prototype.getSplitKeysForChunks = function(chunkSize) {

    var stats = this.stats();

    if (!stats.sharded) {
        print("Collection " + this + " is not sharded.");
        return;
    }

    var config = this.getDB().getSiblingDB("config");

    if (!chunkSize) {
        chunkSize = config.settings.findOne({_id: "chunksize"}).value;
        print("Chunk size not set, using default of " + chunkSize + "MB");
    } else {
        print("Using chunk size of " + chunkSize + "MB");
    }

    var shardDocs = config.shards.find().toArray();

    var allSplitPoints = {};
    var numSplits = 0;

    for (var i = 0; i < shardDocs.length; i++) {
        var shardDoc = shardDocs[i];
        var shard = shardDoc._id;
        var host = shardDoc.host;
        var sconn = new Mongo(host);

        var chunks = config.chunks.find({_id: sh._collRE(this), shard: shard}).toArray();

        print("\nGetting split points for chunks on shard " + shard + " at " + host);

        var splitPoints = [];

        for (var j = 0; j < chunks.length; j++) {
            var chunk = chunks[j];
            var result = sconn.getDB("admin").runCommand(
                {splitVector: this + "", min: chunk.min, max: chunk.max, maxChunkSize: chunkSize});
            if (!result.ok) {
                print(" Had trouble getting split keys for chunk " + sh._pchunk(chunk) + " :\n");
                printjson(result);
            } else {
                splitPoints = splitPoints.concat(result.splitKeys);

                if (result.splitKeys.length > 0)
                    print(" Added " + result.splitKeys.length + " split points for chunk " +
                          sh._pchunk(chunk));
            }
        }

        print("Total splits for shard " + shard + " : " + splitPoints.length);

        numSplits += splitPoints.length;
        allSplitPoints[shard] = splitPoints;
    }

    // Get most recent migration
    var migration = config.changelog.find({what: /^move.*/}).sort({time: -1}).limit(1).toArray();
    if (migration.length == 0)
        print("\nNo migrations found in changelog.");
    else {
        migration = migration[0];
        print("\nMost recent migration activity was on " + migration.ns + " at " + migration.time);
    }

    var admin = this.getDB().getSiblingDB("admin");
    var coll = this;
    var splitFunction = function() {

        // Turn off the balancer, just to be safe
        print("Turning off balancer...");
        config.settings.update({_id: "balancer"}, {$set: {stopped: true}}, true);
        print(
            "Sleeping for 30s to allow balancers to detect change.  To be extra safe, check config.changelog" +
            " for recent migrations.");
        sleep(30000);

        for (var shard in allSplitPoints) {
            for (var i = 0; i < allSplitPoints[shard].length; i++) {
                var splitKey = allSplitPoints[shard][i];
                print("Splitting at " + tojson(splitKey));
                printjson(admin.runCommand({split: coll + "", middle: splitKey}));
            }
        }

        print("Turning the balancer back on.");
        config.settings.update({_id: "balancer"}, {$set: {stopped: false}});
        sleep(1);
    };

    splitFunction.getSplitPoints = function() {
        return allSplitPoints;
    };

    print("\nGenerated " + numSplits + " split keys, run output function to perform splits.\n" +
          " ex : \n" + "  > var splitter = <collection>.getSplitKeysForChunks()\n" +
          "  > splitter() // Execute splits on cluster !\n");

    return splitFunction;

};

DBCollection.prototype.setSlaveOk = function(value) {
    if (value == undefined)
        value = true;
    this._slaveOk = value;
};

DBCollection.prototype.getSlaveOk = function() {
    if (this._slaveOk != undefined)
        return this._slaveOk;
    return this._db.getSlaveOk();
};

DBCollection.prototype.getQueryOptions = function() {
    // inherit this method from DB but use apply so
    // that slaveOk will be set if is overridden on this DBCollection
    return this._db.getQueryOptions.apply(this, arguments);
};

/**
 * Returns a PlanCache for the collection.
 */
DBCollection.prototype.getPlanCache = function() {
    return new PlanCache(this);
};

// Overrides connection-level settings.
//

DBCollection.prototype.setWriteConcern = function(wc) {
    if (wc instanceof WriteConcern) {
        this._writeConcern = wc;
    } else {
        this._writeConcern = new WriteConcern(wc);
    }
};

DBCollection.prototype.getWriteConcern = function() {
    if (this._writeConcern)
        return this._writeConcern;

    if (this._db.getWriteConcern())
        return this._db.getWriteConcern();

    return null;
};

DBCollection.prototype.unsetWriteConcern = function() {
    delete this._writeConcern;
};

//
// CRUD specification read methods
//

/**
* Count number of matching documents in the db to a query.
*
* @method
* @param {object} query The query for the count.
* @param {object} [options=null] Optional settings.
* @param {number} [options.limit=null] The limit of documents to count.
* @param {number} [options.skip=null] The number of documents to skip for the count.
* @param {string|object} [options.hint=null] An index name hint or specification for the query.
* @param {number} [options.maxTimeMS=null] The maximum amount of time to allow the query to run.
* @param {string} [options.readConcern=null] The level of readConcern passed to the count command
* @param {object} [options.collation=null] The collation that should be used for string comparisons
* for this count op.
* @return {number}
*/
DBCollection.prototype.count = function(query, options) {
    query = this.find(query);

    // Apply options and return the result of the find
    return QueryHelpers._applyCountOptions(query, options).count(true);
};

/**
* The distinct command returns returns a list of distinct values for the given key across a
*collection.
*
* @method
* @param {string} key Field of the document to find distinct values for.
* @param {object} query The query for filtering the set of documents to which we apply the distinct
*filter.
* @param {object} [options=null] Optional settings.
* @param {number} [options.maxTimeMS=null] The maximum amount of time to allow the query to run.
* @return {object}
*/
DBCollection.prototype.distinct = function(keyString, query, options) {
    var opts = Object.extend({}, options || {});
    var keyStringType = typeof keyString;
    var queryType = typeof query;

    if (keyStringType != "string") {
        throw new Error("The first argument to the distinct command must be a string but was a " +
                        keyStringType);
    }

    if (query != null && queryType != "object") {
        throw new Error("The query argument to the distinct command must be a document but was a " +
                        queryType);
    }

    // Distinct command
    var cmd = {distinct: this.getName(), key: keyString, query: query || {}};

    // Set maxTimeMS if provided
    if (opts.maxTimeMS) {
        cmd.maxTimeMS = opts.maxTimeMS;
    }

    if (opts.collation) {
        cmd.collation = opts.collation;
    }

    // Execute distinct command
    var res = this.runReadCommand(cmd);
    if (!res.ok) {
        throw new Error("distinct failed: " + tojson(res));
    }

    return res.values;
};

DBCollection.prototype._distinct = function(keyString, query) {
    return this._dbReadCommand({distinct: this._shortName, key: keyString, query: query || {}});
};

DBCollection.prototype.latencyStats = function(options) {
    options = options || {};
    return this.aggregate([{$collStats: {latencyStats: options}}]);
};

/**
 * PlanCache
 * Holds a reference to the collection.
 * Proxy for planCache* commands.
 */
if ((typeof PlanCache) == "undefined") {
    PlanCache = function(collection) {
        this._collection = collection;
    };
}

/**
 * Name of PlanCache.
 * Same as collection.
 */
PlanCache.prototype.getName = function() {
    return this._collection.getName();
};

/**
 * toString prints the name of the collection
 */
PlanCache.prototype.toString = function() {
    return "PlanCache for collection " + this.getName() + '. Type help() for more info.';
};

PlanCache.prototype.shellPrint = PlanCache.prototype.toString;

/**
 * Displays help for a PlanCache object.
 */
PlanCache.prototype.help = function() {
    var shortName = this.getName();
    print("PlanCache help");
    print("\tdb." + shortName + ".getPlanCache().help() - show PlanCache help");
    print("\tdb." + shortName + ".getPlanCache().listQueryShapes() - " +
          "displays all query shapes in a collection");
    print("\tdb." + shortName + ".getPlanCache().clear() - " +
          "drops all cached queries in a collection");
    print("\tdb." + shortName +
          ".getPlanCache().clearPlansByQuery(query[, projection, sort, collation]) - " +
          "drops query shape from plan cache");
    print("\tdb." + shortName +
          ".getPlanCache().getPlansByQuery(query[, projection, sort, collation]) - " +
          "displays the cached plans for a query shape");
    return __magicNoPrint;
};

/**
 * Internal function to parse query shape.
 */
PlanCache.prototype._parseQueryShape = function(query, projection, sort, collation) {
    if (query == undefined) {
        throw new Error("required parameter query missing");
    }

    // Accept query shape object as only argument.
    // Query shape must contain 'query', 'projection', and 'sort', and may optionally contain
    // 'collation'. 'collation' must be non-empty if present.
    if (typeof(query) == 'object' && projection == undefined && sort == undefined &&
        collation == undefined) {
        var keysSorted = Object.keys(query).sort();
        // Expected keys must be sorted for the comparison to work.
        if (bsonWoCompare(keysSorted, ['projection', 'query', 'sort']) == 0) {
            return query;
        }
        if (bsonWoCompare(keysSorted, ['collation', 'projection', 'query', 'sort']) == 0) {
            if (Object.keys(query.collation).length === 0) {
                throw new Error("collation object must not be empty");
            }
            return query;
        }
    }

    // Extract query shape, projection, sort and collation from DBQuery if it is the first
    // argument. If a sort or projection is provided in addition to DBQuery, do not
    // overwrite with the DBQuery value.
    if (query instanceof DBQuery) {
        if (projection != undefined) {
            throw new Error("cannot pass DBQuery with projection");
        }
        if (sort != undefined) {
            throw new Error("cannot pass DBQuery with sort");
        }
        if (collation != undefined) {
            throw new Error("cannot pass DBQuery with collation");
        }

        var queryObj = query._query["query"] || {};
        projection = query._fields || {};
        sort = query._query["orderby"] || {};
        collation = query._query["collation"] || undefined;
        // Overwrite DBQuery with the BSON query.
        query = queryObj;
    }

    var shape = {
        query: query,
        projection: projection == undefined ? {} : projection,
        sort: sort == undefined ? {} : sort,
    };

    if (collation !== undefined) {
        shape.collation = collation;
    }

    return shape;
};

/**
 * Internal function to run command.
 */
PlanCache.prototype._runCommandThrowOnError = function(cmd, params) {
    var res = this._collection.runCommand(cmd, params);
    if (!res.ok) {
        throw new Error(res.errmsg);
    }
    return res;
};

/**
 * Lists query shapes in a collection.
 */
PlanCache.prototype.listQueryShapes = function() {
    return this._runCommandThrowOnError("planCacheListQueryShapes", {}).shapes;
};

/**
 * Clears plan cache in a collection.
 */
PlanCache.prototype.clear = function() {
    this._runCommandThrowOnError("planCacheClear", {});
    return;
};

/**
 * List plans for a query shape.
 */
PlanCache.prototype.getPlansByQuery = function(query, projection, sort, collation) {
    return this
        ._runCommandThrowOnError("planCacheListPlans",
                                 this._parseQueryShape(query, projection, sort, collation))
        .plans;
};

/**
 * Drop query shape from the plan cache.
 */
PlanCache.prototype.clearPlansByQuery = function(query, projection, sort, collation) {
    this._runCommandThrowOnError("planCacheClear",
                                 this._parseQueryShape(query, projection, sort, collation));
    return;
};