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Polymorphs

Polymorphs allow a single field to return different object types based on runtime conditions. This is useful when a field could contain one of several related but distinct types.

Defining a Polymorph

module MyAPI
  module Objects
    class EventSubject < Apia::Polymorph

      name "Event Subject"
      description "The subject of an event (could be a user, organization, or server)"

      option :user, type: Objects::User, matcher: -> (value) { value.is_a?(::User) }
      option :organization, type: Objects::Organization, matcher: -> (value) { value.is_a?(::Organization) }
      option :server, type: Objects::Server, matcher: -> (value) { value.is_a?(::Server) }

    end
  end
end

DSL Methods

option(name, type:, matcher:)

Adds a polymorph option. Each option defines:

  • name - A symbol identifying this option
  • type: - The object type (or scalar/enum) to serialize the value as
  • matcher: - A lambda that returns true if the given value should use this option
option :user, type: Objects::User, matcher: -> (value) { value.is_a?(::User) }
option :organization, type: Objects::Organization, matcher: -> (value) { value.is_a?(::Organization) }

Using in Fields

Reference the polymorph type on a field:

class Event < Apia::Object
  field :subject, Objects::EventSubject
end

Response Format

When a polymorph field is serialized, the response includes the matched type name and the serialized value:

{
  "subject": {
    "type": "user",
    "value": {
      "id": "abc-123",
      "name": "John Doe",
      "email": "john@example.com"
    }
  }
}

How It Works

  1. When serializing a polymorph field, Apia iterates through the options in order
  2. For each option, it calls the matcher lambda with the actual value
  3. The first option whose matcher returns true is used
  4. The value is then serialized using that option's type
  5. If no option matches, an InvalidPolymorphValueError is raised

Complete Example

Define the types

class UserObject < Apia::Object
  field :id, :string
  field :name, :string
  field :email, :string
end

class TeamObject < Apia::Object
  field :id, :string
  field :name, :string
  field :member_count, :integer
end

Define the polymorph

class Assignable < Apia::Polymorph
  name "Assignable"
  description "Something that can be assigned to a task"

  option :user, type: UserObject, matcher: -> (v) { v.is_a?(::User) }
  option :team, type: TeamObject, matcher: -> (v) { v.is_a?(::Team) }
end

Use in an object

class Task < Apia::Object
  field :id, :string
  field :title, :string
  field :assignee, Assignable, null: true
end