GraphQL APIs with graphql-ruby gem

# app/graphql/mutations/create_post.rb
module Mutations
  class CreatePost < BaseMutation
    description "Create a new post"

    # Input arguments
    argument :title, String, required: true
    argument :content, String, required: true
    argument :published, Boolean, required: false, default_value: false

    # Return fields
    field :post, Types::PostType, null: true
    field :errors, [String], null: false

    def resolve(title:, content:, published:)
      user = context[:current_user]

      unless user
        return {
          post: nil,
          errors: ["Must be logged in"]
        }
      end

      post = user.posts.build(
        title: title,
        content: content,
        published: published
      )

      if post.save
        {
          post: post,
          errors: []
        }
      else
        {
          post: nil,
          errors: post.errors.full_messages
        }
      end
    end
  end
end

# app/graphql/mutations/update_post.rb
module Mutations
  class UpdatePost < BaseMutation
    argument :id, ID, required: true
    argument :title, String, required: false
    argument :content, String, required: false
    argument :published, Boolean, required: false

    field :post, Types::PostType, null: true
    field :errors, [String], null: false

    def resolve(id:, **attributes)
      post = Post.find(id)
      user = context[:current_user]

      unless user&.id == post.user_id
        return {
          post: nil,
          errors: ["Not authorized"]
        }
      end

      if post.update(attributes.compact)
        { post: post, errors: [] }
      else
        { post: nil, errors: post.errors.full_messages }
      end
    end
  end
end

# app/graphql/types/mutation_type.rb
module Types
  class MutationType < Types::BaseObject
    field :create_post, mutation: Mutations::CreatePost
    field :update_post, mutation: Mutations::UpdatePost
    field :delete_post, mutation: Mutations::DeletePost
  end
end

# GraphQL controller
class GraphqlController < ApplicationController
  def execute
    variables = prepare_variables(params[:variables])
    query = params[:query]
    operation_name = params[:operationName]

    context = {
      current_user: current_user,
      pundit_user: current_user
    }

    result = MyAppSchema.execute(
      query,
      variables: variables,
      context: context,
      operation_name: operation_name
    )

    render json: result
  rescue StandardError => e
    raise e unless Rails.env.development?

    handle_error_in_development(e)
  end

  private

  def prepare_variables(variables_param)
    case variables_param
    when String
      JSON.parse(variables_param) || {}
    when Hash
      variables_param
    when ActionController::Parameters
      variables_param.to_unsafe_hash
    when nil
      {}
    else
      raise ArgumentError, "Unexpected variables: #{variables_param}"
    end
  end
end

# Example GraphQL queries
# Query single user:
# {
#   user(id: 1) {
#     id
#     name
#     email
#     postsCount
#     posts {
#       id
#       title
#       excerpt
#     }
#   }
# }

# Mutation to create post:
# mutation {
#   createPost(input: {
#     title: "My Post"
#     content: "Post content here"
#     published: true
#   }) {
#     post {
#       id
#       title
#     }
#     errors
#   }
# }

# Gemfile
gem 'graphql-batch'

# Setup in schema
class MyAppSchema < GraphQL::Schema
  use GraphQL::Batch
end

# app/graphql/loaders/record_loader.rb
class RecordLoader < GraphQL::Batch::Loader
  def initialize(model)
    @model = model
  end

  def perform(ids)
    records = @model.where(id: ids).index_by(&:id)
    ids.each { |id| fulfill(id, records[id]) }
  end
end

# app/graphql/loaders/association_loader.rb
class AssociationLoader < GraphQL::Batch::Loader
  def initialize(model, association_name)
    @model = model
    @association_name = association_name
  end

  def perform(records)
    # Preload association for all records at once
    records_with_association = @model
      .where(id: records.map(&:id))
      .includes(@association_name)
      .index_by(&:id)

    records.each do |record|
      record_with_association = records_with_association[record.id]
      fulfill(record, record_with_association.public_send(@association_name))
    end
  end
end

# Using loaders in types
module Types
  class UserType < Types::BaseObject
    field :posts, [Types::PostType], null: false

    def posts
      AssociationLoader.for(User, :posts).load(object)
    end
  end

  class PostType < Types::BaseObject
    field :user, Types::UserType, null: false

    def user
      RecordLoader.for(User).load(object.user_id)
    end

    field :comments, [Types::CommentType], null: false

    def comments
      AssociationLoader.for(Post, :comments).load(object)
    end
  end
end

# This batches database queries:
# Without loader: N+1 queries
#   posts.each { |post| post.user }  # Queries for each post

# With loader: 2 queries total
#   SELECT * FROM posts
#   SELECT * FROM users WHERE id IN (1,2,3,4,5)

# Custom count loader
class CountLoader < GraphQL::Batch::Loader
  def initialize(association_name, scope = nil)
    @association_name = association_name
    @scope = scope
  end

  def perform(records)
    counts = records
      .group_by(&:class)
      .flat_map { |klass, records|
        scope = klass.joins(@association_name)
          .where(id: records.map(&:id))
          .group("#{klass.table_name}.id")

        scope = scope.merge(@scope) if @scope

        scope.count
      }

    records.each do |record|
      fulfill(record, counts[record.id] || 0)
    end
  end
end

# Usage
field :published_posts_count, Integer, null: false

def published_posts_count
  CountLoader
    .for(:posts, Post.where(published: true))
    .load(object)
end