Polymorphic types with GraphQL-ruby

Polymorphic associations on Ruby on Rails Guide

Problem statement

Polymorphic associations are most often used in a Rails application. With GraphQL ruby one of the problems we come across is to How to resolve polymorphic types in GraphQL-ruby?.

The solution is to use Union types. To understand more on how to resolve polymorphic types lets take the following example of Polymorphic association from Ruby on Rails Guide.

class Picture < ApplicationRecord
  belongs_to :imageable, polymorphic: true
end

class Employee < ApplicationRecord
  has_many :pictures, as: :imageable
end

class Product < ApplicationRecord
  has_many :pictures, as: :imageable
end

In the above example, imageable Polymorphic association for PictureType can be either an Employee or Product as imageable. Employee and Product may have different properties and to resolve the different properties, the GraphQL type should know what type of fields the object would resolve. This is where Union Types come into the picture.

Definition from GraphQL-ruby guides

What is Union Types?

A union type is a set of object types which may appear in the same spot.

// apps/graphql/types/picture_type.rb
class Types::PictureType < Types::BaseObject
  description "Properties of Picture"

  field :id, ID, null: false
  field :imageable, Types::ImageType, null: false
end

Now, add a ImageType which may either resolve ProductType or EmployeeType.

class Types::ImageType < Types::BaseUnion
  description "Properties of Image"

  possible_types Types::EmployeeType,
                 Types::ProductType,

  def self.resolve_type(object, context)
    if object.is_a?(Employee)
      Types::EmployeeType
    elsif object.is_a?(Product)
      Types::ProductType
    end
  end
end

The GraphQL query would now be written as follows.

pictures {
  id
  ... on Employee {
    first_name
    last_name
    address
  }
  ... on Product {
    name
    price
  }
}

Happy Coding!!