JSON Interoperability Guide

It is often useful to convert data that was retrieved from MongoDB to JSON, either for producing a human readable version of it or for serving it up via a REST API. BSON (the format that MongoDB uses to store data) supports more types than JSON does though, which means JSON alone can’t represent BSON data losslessly. To solve this issue, you can convert your data to Extended JSON, which is a standard format of JSON used by the various drivers to represent BSON data in JSON that includes extra information indicating the BSON type of a given value. If preserving the type information isn’t required, then Foundation’s JSONEncoder and JSONDecoder can be used to convert the data to regular JSON, though not all BSON types currently support working with them (e.g. BSONBinary).

Extended JSON

As mentioned above, Extended JSON is a form of JSON that preserves type information. There are two forms of extended JSON, and the form used determines how much extra type information is included in the JSON format for a given type.

The two formats of extended JSON are as follows:

  • Relaxed Extended JSON - A string format based on the JSON standard that describes BSON documents. Relaxed Extended JSON emphasizes readability and interoperability at the expense of type preservation.
    • example: {"d": 5.5}
  • Canonical Extended JSON - A string format based on the JSON standard that describes BSON documents. Canonical Extended JSON emphasizes type preservation at the expense of readability and interoperability.
    • example: {"d": {"$numberDouble": 5.5}}

Here we can see the same data: a key, "i" with the value 1 represented in BSON, and two forms of Extended JSON


// Relaxed Extended JSON
{"i": 1}

// Canonical Extended JSON
{"i": {"$numberInt":"1"}}

To see how all of the BSON types are represented in Canonical and Relaxed Extended JSON Format, see the documentation here.

A thorough example Canonical Extended JSON document and its relaxed counterpart can be found here.

Generating and Parsing Extended JSON via Codable

The ExtendedJSONEncoder and ExtendedJSONDecoder provide a way for any custom Codable classes to interact with canonical or relaxed extended JSON. They can be used just like JSONEncoder and JSONDecoder.

let encoder = ExtendedJSONEncoder()
let decoder = ExtendedJSONDecoder()

struct Person: Codable, Equatable {
    let name: String
    let age: Int32

let bobExtJSON = try encoder.encode(Person(name: "Bob", age: 25)) // "{\"name\":\"Bob\",\"age\":25}}"
let joe = try decoder.decode(Person.self, from: "{\"name\":\"Joe\",\"age\":34}}".data(using: .utf8)!)

The ExtendedJSONEncoder produces relaxed Extended JSON by default, but can be configured to produce canonical.

let bob = Person(name: "Bob", age: 25)
let encoder = ExtendedJSONEncoder()
encoder.format = .canonical
let canonicalEncoded = try encoder.encode(bob) // "{\"name\":\"Bob\",\"age\":{\"$numberInt\":\"25\"}}"

The ExtendedJSONDecoder accepts either format, or a mix of both:

let decoder = ExtendedJSONDecoder()

let canonicalExtJSON = "{\"name\":\"Bob\",\"age\":{\"$numberInt\":\"25\"}}"
let canonicalDecoded = try decoder.decode(Person.self, from: canonicalExtJSON.data(using: .utf8)!) // bob

let relaxedExtJSON = "{\"name\":\"Bob\",\"age\":25}}"
let relaxedDecoded = try decoder.decode(Person.self, from: relaxedExtJSON.data(using: .utf8)!) // bob

Using Extended JSON with Vapor

By default, Vapor uses JSONEncoder and JSONDecoder for encoding and decoding its Content to and from JSON. If you are interested in using the ExtendedJSONEncoder and ExtendedJSONDecoder in your Vapor app instead, you can set them as the default encoder and decoder and thereby allow your application to serialize and deserialize data to/from Extended JSON, rather than the default plain JSON. This is recommended because not all BSON types currently support working with JSONEncoder and JSONDecoder and also so that you can take advantage of the added type information. From the Vapor Documentation: you can set the global configuration and change the encoders and decoders Vapor uses by default by doing something like this:

let encoder = ExtendedJSONEncoder()
let decoder = ExtendedJSONDecoder()
ContentConfiguration.global.use(encoder: encoder, for: .json)
ContentConfiguration.global.use(decoder: decoder, for: .json)

in your configure.swift.

Note that in order for this to work, ExtendedJSONEncoder must conform to Vapor’s ContentEncoder protocol, and ExtendedJSONDecoder must conform to Vapor’s ContentDecoder protocol.

We’ve added these conformances in our Vapor integration library MongoDBVapor, which we highly recommend using if you want to use the driver with Vapor.

To see an example Vapor app using the driver via the integration library, check out Examples/VaporExample.

Using JSONEncoder and JSONDecoder with BSON Types

Currently, some BSON types (e.g. BSONBinary) do not support working with encoders and decoders other than those introduced in swift-bson, meaning Foundation’s JSONEncoder and JSONDecoder will throw errors when encoding or decoding such types. There are plans to add general Codable support for all BSON types in the future, though. For now, only BSONObjectID and any BSON types defined in Foundation or the standard library (e.g. Date or Int32) will work with other encoder/decoder pairs. If type information is not required in the output JSON and only types that include a general Codable conformance are included in your data, you can use JSONEncoder and JSONDecoder to produce and ingest JSON data.

let foo = Foo(x: BSONObjectID(), date: Date(), y: 3.5)
try JSONEncoder().encode(foo) // "{\"x\":<hexstring>,\"date\":<seconds since reference date>,\"y\":3.5}"