Distinct

[mcnamee]

Hi, is it possible to get 'distinct' records through a rest call (e.g. With the mongoose distinct())? Is possible to Perhaps add it via a before hook?

E.g. If I have a collection full of books, and I want to get just the names of all distinct Authors

[daffl]

You could add your own query parameter and make the model call based on that query in the hook, setting hook.result similar to this:

app.service('users').before({
  find(params) {
    if(params.query.$distinct) {
      return this.Model.find().distinct().then(data => {
        hook.result = data;
        return hook;
      });
    }
  }
});

[mcnamee]

Thanks so much for the quick response @daffl - By doing it this way, is my understanding correct that it would do 2 DB queries, where the second is unneeded?

[daffl]

No. If you set the result in a before hook the original database call is not going to happen.

[mcnamee]

Wow, great to know. Thanks again!
I'll give it a go

[mcnamee]

Hi @daffl - I ended up creating a hook like this, which seems to work a treat:

/*
 * Distinct Search - can only be 1 distinct at a time
 * - allows you to URI query like: books?$distinct=author
 * ============================================================
 */
exports.searchDistinct = function () {
  return function (hook) {
    let query = hook.params.query;

    // Must be a before hook
    if (hook.type !== 'before') {
      throw new Error('The \'searchDistinct\' hook should only be used as a \'before\' hook (hooks.searchDistinct).');
    }

    // Throw error when no field is provided - eg. just users?$distinct
    if (query.$distinct === '') {
     throw new Error('Missing $distinct: Which field should be distinct? (hooks.searchDistinct)');
    }

    let distinctValue = query.$distinct || null;
    if (distinctValue == null) return hook;

    // Remove $distinct param from query (preventing errors)
    delete query.$distinct;

    return new Promise((resolve, reject) => {
      var args = [
        { $match: query || {} },
        { $group: {
          _id: "$" + distinctValue,
          total: { $sum: 1 }
        }}
      ];
      this.Model.aggregate(args)
        .then(data => {
          hook.result = {
            total: data.length || 0,
            data,
          };
          resolve();
        }).catch(err => reject(err));
    });
  }
}

This allows you to hit an endpoint like /users?status=active&$distinct=country and returns data similar to:

{
  "total": 25,
  "data": [
    {
      "_id": "Spain",
      "total": 22
    },
    {
      "_id": "Denmark",
      "total": 12
    }
   ...

[daffl]

Awesome, thank you for sharing!

Oh, btw, you can probably just return this.Model.aggregate(args) instead of having to create a wrapper promise.

[mcnamee]

Thanks @daffl - that's true - although I have the wrapper promise so that I can alter the hook.result to include a total and if needed, later on extend to include $skip and $limit for example.

[daffl]

What I meant is that you can write the same thing like this:

/*
 * Distinct Search - can only be 1 distinct at a time
 * - allows you to URI query like: books?$distinct=author
 * ============================================================
 */
exports.searchDistinct = function () {
  return function (hook) {
    let query = hook.params.query;

    // Must be a before hook
    if (hook.type !== 'before') {
      throw new Error('The \'searchDistinct\' hook should only be used as a \'before\' hook (hooks.searchDistinct).');
    }

    // Throw error when no field is provided - eg. just users?$distinct
    if (query.$distinct === '') {
     throw new Error('Missing $distinct: Which field should be distinct? (hooks.searchDistinct)');
    }

    let distinctValue = query.$distinct || null;
    if (distinctValue == null) return hook;

    // Remove $distinct param from query (preventing errors)
    delete query.$distinct;

    var args = [
      { $match: query || {} },
      { $group: {
        _id: "$" + distinctValue,
        total: { $sum: 1 }
      }}
    ];

    return this.Model.aggregate(args)
      .then(data => {
        hook.result = {
          total: data.length || 0,
          data,
        };

        return hook;
      });
  }
}

I found that especially when dealing with other calls that return a promise there is usually no need to create a new promise with new Promise((resolve, reject) => {})

[mcnamee]

Ah hah! Very helpful, thank you!

[ekryski]

I'm going to try and keep this in mind for reference in case we want to roll this into some common hooks. cc/ @feathersjs/core-team

[eddyystop]

I've tagged this as a todo for myself.

Please comment on any enhancements, changes, etc you have in mind.

[vigalo]

Hello
I need to create a hook like this one to rethinkdb database adapter. How can I do it?
thanks.

[vigalo]

I solved my problem like this.

let query = hook.params.query;

// Must be a before hook
if (hook.type !== 'before') {
  throw new Error('El hook \'distinct\' sólo puede usarse como hook \'before\' (hooks.distinct).');
}

// Throw error when no field is provided - eg. just users?$distinct
if (query.$distinct === '') {
  throw new Error('$distinct no encontrado: ¿Qué campo debería ser distinto? (hooks.distinct)');
}

let distinctValue = query.$distinct || null;
console.log('distinctValue', distinctValue);
if (distinctValue == null) return hook;

// Remove $distinct param from query (preventing errors)
delete query.$distinct;

// Add only the field we are searching for
query = Object.assign({ $select: distinctValue },query)

const rethinkdbQuery = this.createQuery(query);
const r = this.options.r;

// In our case, disable pagination
hook.params.paginate = false;

// Update the query with an additional `distinct` condition
hook.params.rethinkdb = rethinkdbQuery.distinct();

return hook;