module JSON::Serializable

Overview

The JSON::Serializable module automatically generates methods for JSON serialization when included.

Example

require "json"

class Location
  include JSON::Serializable

  @[JSON::Field(key: "lat")]
  property latitude : Float64

  @[JSON::Field(key: "lng")]
  property longitude : Float64
end

class House
  include JSON::Serializable
  property address : String
  property location : Location?
end

house = House.from_json(%({"address": "Crystal Road 1234", "location": {"lat": 12.3, "lng": 34.5}}))
house.address  # => "Crystal Road 1234"
house.location # => #<Location:0x10cd93d80 @latitude=12.3, @longitude=34.5>
house.to_json  # => %({"address":"Crystal Road 1234","location":{"lat":12.3,"lng":34.5}})

houses = Array(House).from_json(%([{"address": "Crystal Road 1234", "location": {"lat": 12.3, "lng": 34.5}}]))
houses.size    # => 1
houses.to_json # => %([{"address":"Crystal Road 1234","location":{"lat":12.3,"lng":34.5}}])

Usage

Including JSON::Serializable will create #to_json and self.from_json methods on the current class, and a constructor which takes a JSON::PullParser. By default, these methods serialize into a json object containing the value of every instance variable, the keys being the instance variable name. Most primitives and collections supported as instance variable values (string, integer, array, hash, etc.), along with objects which define to_json and a constructor taking a JSON::PullParser. Union types are also supported, including unions with nil. If multiple types in a union parse correctly, it is undefined which one will be chosen.

To change how individual instance variables are parsed and serialized, the annotation JSON::Field can be placed on the instance variable. Annotating property, getter and setter macros is also allowed.

require "json"

class A
  include JSON::Serializable

  @[JSON::Field(key: "my_key", emit_null: true)]
  getter a : Int32?
end

JSON::Field properties:

Deserialization also respects default values of variables:

require "json"

struct A
  include JSON::Serializable
  @a : Int32
  @b : Float64 = 1.0
end

A.from_json(%<{"a":1}>) # => A(@a=1, @b=1.0)

Extensions: JSON::Serializable::Strict and JSON::Serializable::Unmapped.

If the JSON::Serializable::Strict module is included, unknown properties in the JSON document will raise a parse exception. By default the unknown properties are silently ignored. If the JSON::Serializable::Unmapped module is included, unknown properties in the JSON document will be stored in a Hash(String, JSON::Any). On serialization, any keys inside json_unmapped will be serialized and appended to the current json object.

require "json"

struct A
  include JSON::Serializable
  include JSON::Serializable::Unmapped
  @a : Int32
end

a = A.from_json(%({"a":1,"b":2})) # => A(@json_unmapped={"b" => 2_i64}, @a=1)
a.to_json                         # => {"a":1,"b":2}

Class annotation JSON::Serializable::Options

supported properties:

require "json"

@[JSON::Serializable::Options(emit_nulls: true)]
class A
  include JSON::Serializable
  @a : Int32?
end

Discriminator field

A very common JSON serialization strategy for handling different objects under a same hierarchy is to use a discriminator field. For example in GeoJSON each object has a "type" field, and the rest of the fields, and their meaning, depend on its value.

You can use JSON::Serializable.use_json_discriminator for this use case.

after_initialize method

#after_initialize is a method that runs after an instance is deserialized from JSON. It can be used as a hook to post-process the initialized object.

Example:

require "json"

class Person
  include JSON::Serializable
  getter name : String

  def after_initialize
    @name = @name.upcase
  end
end

person = Person.from_json %({"name": "Jane"})
person.name # => "JANE"

Defined in:

json/serialization.cr

Constructors

Instance Method Summary

Macro Summary

Constructor Detail

def self.new(*, __pull_for_json_serializable pull : JSON::PullParser) #

[View source]

Instance Method Detail

def to_json(json : JSON::Builder) #

[View source]

Macro Detail

macro use_json_discriminator(field, mapping) #

Tells this class to decode JSON by using a field as a discriminator.

  • field must be the field name to use as a discriminator
  • mapping must be a hash or named tuple where each key-value pair maps a discriminator value to a class to deserialize

For example:

require "json"

abstract class Shape
  include JSON::Serializable

  use_json_discriminator "type", {point: Point, circle: Circle}

  property type : String
end

class Point < Shape
  property x : Int32
  property y : Int32
end

class Circle < Shape
  property x : Int32
  property y : Int32
  property radius : Int32
end

Shape.from_json(%({"type": "point", "x": 1, "y": 2}))               # => #<Point:0x10373ae20 @type="point", @x=1, @y=2>
Shape.from_json(%({"type": "circle", "x": 1, "y": 2, "radius": 3})) # => #<Circle:0x106a4cea0 @type="circle", @x=1, @y=2, @radius=3>

[View source]