class Books
 belongs_to :user
end

class Users
 has_many :books
end

first_name
last_name
email

author
title
description

Types::BookType = GraphQL::ObjectType.define do
 name 'Book'
field :id, !types.ID
field :author, !types.String
field :title, !types.String
field :user, -> {Types::UserType}, property: :user
end

Types::UserType = GraphQL::ObjectType.define do
 name 'User'
 field :id, !types.ID
 field :first_name, !types.String
 field :last_name, !types.String
 field :email, !types.String
end

Types::MutationType = GraphQL::ObjectType.define do
 name 'Mutation'
 field :bookCreate, function: Resolvers::BookCreate.new
end

class Resolvers::BookCreate < GraphQL::Function
 argument :author, !types.String
 argument :title, !types.String
 argument :description, !types.String
 argument :user, !types.Int

 type Types::BookType

 def call(_obj, args, _ctx)
  Book.create!(
   author: args[:author],
   title: args[:title],
   description: args[:description],
   user: args[:user]
   )
 end
end

mutation book {
 bookCreate(
  author: "Test",
  title: "Book Test",
  description: "Book Description",
  user: 2) {
 id
 author
 title
 description
 }
}

<ActiveRecord::AssociationTypeMismatch: User(#69982231587660) expected, got 2 which is an instance of Integer(#11387600)>