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:

• ignore: if true skip this field in serialization and deserialization (by default false)
• ignore_serialize: If truthy, skip this field in serialization (default: false). The value can be any Crystal expression and is evaluated at runtime.
• ignore_deserialize: if true skip this field in deserialization (by default false)
• key: the value of the key in the json object (by default the name of the instance variable)
• root: assume the value is inside a JSON object with a given key (see Object.from_json(string_or_io, root))
• converter: specify an alternate type for parsing and generation. The converter must define from_json(JSON::PullParser) and to_json(value, JSON::Builder). Examples of converters are a Time::Format instance and Time::EpochConverter for Time.
• presence: if true, a @{{key}}_present instance variable will be generated when the key was present (even if it has a null value), false by default
• emit_null: if true, emits a null value for nilable property (by default nulls are not emitted)

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}, @a=1)
a.json_unmapped["b"].raw.class    # => Int64
a.to_json                         # => %({"a":1,"b":2})

Class annotation JSON::Serializable::Options

supported properties:

• emit_nulls: if true, emits a null value for all nilable properties (by default nulls are not emitted)

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:

macros.cr