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Introduction to bson

ver 1.1.2

nimble repo.support

BSON

This is a Nim library for supporting BSON. BSON is short for Binary JSON -- a compact binary protocol similar to the JSON it is based on.

Most notably, MongoDB, a document-oriented database uses BSON for it's underlying storage, though there are other applications that use it as well.

More details about the protocol can be found at:

http://bsonspec.org/

More detail can also be found in the reference document linked at the bottom.

CREATING A BSON DOCUMENT USING BRACKETS

You can use a pair of '@@' symbols prefixing a pair of curly braces with json-like data in between to create a quick an easy BSON document.

For example:

var doc = @@{"name": "Joe", "age": 42, "siblings": ["Amy", "Jerry"]}

CREATING A BSON DOCUMENT MANUALLY

Start building a BSON document by declaring a new document with newBsonDocument and start building out items in that document as if were using a table.

var doc = newBsonDocument()
doc["name"] = "Joe"
doc["age"] = 42

You can also add sub elements such as lists (with newBsonArray) and other documents (with newBsonDocument):

doc["siblings"] = newBsonArray()
doc["siblings"].add "Amy"
doc["siblings"].add "Jerry"
doc["schedule"] = newBsonDocument()
doc["schedule"]["8am"] = "go to work"
doc["schedule"]["11am"] = "see dentist"

READING A BSON DOCUMENT

To read a BSON document, you can reference the field by string in either traditional square brackets ([]) or the forgiving curly brackets ({}).

var doc = @@{
  "name": "Joe",
  "address": {"city": "New Orleans", "state": "LA"},
  "pots": [9, 22, 16]
}

let personName = doc["name"]                 # set to "Joe"
let personState = doc["address"]["state"]    # set to "LA"
let secondPot = doc["pots"][1]               # set to 22

When using square brackets, if the key is missing a runtime error is generated. But when using curly brackets, a missing key simply results in a null. And, the keys can be separated by commas to easily transverse down the tree.

let personCity = doc{"address", "city"}      # set to "New Orleans"
let personCode = doc{"address", "postal"}    # set to null
let thirdPot = doc{"pots", "2"}              # set to 16
let fourthPot = doc{"pots", "3"}             # set to null

GENERATING THE BSON CODE

To generate the actual binary code, such as to stream to a file or a service, use the 'bytes' function:

var bString: string = doc.bytes()

Please keep in mind that this is a binary packed string and is not printable.

To convert a binary blob of data back into a Bson library document, pass the string into 'newBsonDocument' as a string parameter.

var newDoc = newBsonDocument(bString)

HANDLING TYPES

The BSON specification calls for 18 types of data (and a few subtypes).

Not all of them are fully supported by the libary yet.

BSON Nim Equiv Notes
64-bit binary floating point float Nim defaults to 64 bit
UTF-8 string string Nim strings are UTF-8 ready by default
Embedded document newBsonDocument from this library. for key/value pairs, the key must always be a string
Array newBsonArray technically a list, not an array, because you can mix types
Binary data string (binary) not always printable, but works, see binstr
ObjectId Oid from standard oids library
Boolean "false" bool = false  
Boolean "true" bool = true  
UTC datetime Time from standard times library
Null value null from this library
Regular expression regex() from this library
DBPointer (deprecated) dbref() from this library
JavaScript code js() from this library
JavaScript code w/ scope    
32-bit integer int32  
Timestamp BsonTimestamp from this library. Do not use to store dates or time. Meant for internal use by MongoDb only.
64-bit integer int64  
128-bit decimal floating point   would like to support !
Min key    
Max key    

Marshal

There is a submodule called marshal, that allows for the easy conversion of object types to/from BSON. It has a single macro: marshal which generates toBson and pull procedure for the object.

An example:

import bson
import bson/marshal

type
  User = object
    name: string
    height: Option[float]

marshal(User)

var u = User()

var someBson = @@{"name": "Bob", "height": 95.3}

u.pull(someBson)

assert u.name == "Bob"

See the bson/marshal Reference link in the Table of Contents below for more detail.

Credit

Large portions of this code were pulled from the nimongo project, a scalable pure-nim mongodb driver. See https://github.com/SSPkrolik/nimongo

However, this library is NOT compatilible with nimongo, as nimongo relies on an internal implementation of BSON.

Table Of Contents

  1. Introduction to bson

  2. Appendices

    1. bson Reference
    2. bson/marshal Reference