install

# Gemfile

gem 'graphql'

$ bundle

models

users	type
name	String
age	Integer

app/models/user.rb

class User < ApplicationRecord
  def friends
    %i[user mate_user].inject(Friend.none) do |friends_query, column_name|
      friends_query.or(Friend.where(column_name => self))
    end
  end
end

friends	type
user_id	Integer
mate_user_id	Integer

app/models/friend.rb

class Friend < ApplicationRecord
  belongs_to :user
  belongs_to :mate_user, class_name: :User

  def pairs
    [user, mate_user]
  end
end

friend model has two users.

setup

app/graphql/graphql_rails_minimum_sample_schema.rb

class GraphqlRailsMinimumSampleSchema < GraphQL::Schema
  query(GqlTypes::Query)
end

app/graphql/gql_types.rb

module GqlTypes
  class Query < GraphQL::Schema::Object
    field :users,   [UserType],   null: false
    field :friends, [FriendType], null: false do
      argument :user_id, Int, required: true, camelize: false
    end

    def users
      User.all
    end

    def friends(user_id:)
      User.find(user_id).friends
    end
  end
end

app/graphql/gql_types/base_object.rb

module GqlTypes
  class BaseObject < GraphQL::Schema::Object
  end
end

app/graphql/gql_types/friend_type.rb

module GqlTypes
  class FriendType < BaseObject
    field :pairs, [UserType], null: false
  end
end

app/graphql/gql_types/user_type.rb

module GqlTypes
  class UserType < BaseObject
    field :name,       String,       null: false
    field :age,        String,       null: true
    field :created_at, String,       null: false
    field :friends,    [FriendType], null: false
  end
end

execute query!!

注意点

created_atなどsnake caseのデータ名はデフォルトでcamel caseで指定する
camelize false を指定すればsnake caseで指定できる
argumentはvariablesに渡す
トップレベルの{}に注意(以下)

# queryには{}をつけない
query get_friends($user_id: Int!) {
  # camelize falseなためuserIdではない(:point_up_2: :point_down: )
  friends(user_id: $user_id) {}
}

{
  users {}
}

try

rails c

> GraphqlRailsMinimumSampleSchema.execute('{ users { name createdAt age friends { pairs { name } } } }').to_h
=> {"data"=>{
  "users"=>[{
    "name"=>"nishi",
    "createdAt"=>"2019-04-14 04:01:37 UTC",
    "age"=>"9",
    "friends"=>[{
      "pairs"=>[{"name"=>"nishi"}, {"name"=>"tarou"}]
    }]
  }, {
    "name"=>"suke",
    "createdAt"=>"2019-04-14 04:01:37 UTC",
    "age"=>"19",
    "friends"=>[{
      "pairs"=>[{"name"=>"suke"}, {"name"=>"tarou"}]
    }]
  }, {
    "name"=>"tarou",
    "createdAt"=>"2019-04-14 04:01:37 UTC",
    "age"=>"20",
    "friends"=>[
      {"pairs"=>[{"name"=>"nishi"}, {"name"=>"tarou"}]},
      {"pairs"=>[{"name"=>"suke"},  {"name"=>"tarou"}]}
    ]
  }]
}}

> GraphqlRailsMinimumSampleSchema.execute('query get_friends($user_id: Int!) { friends(user_id: $user_id) { pairs { name } } }', variables: { user_id: User.last.id } ).to_json
=> {"data"=>{"friends"=>[
  {"pairs"=>[{"name"=>"nishi"}, {"name"=>"tarou"}]},
  {"pairs"=>[{"name"=>"suke"}, {"name"=>"tarou"}]}
]}}