Rails GraphQL Patterns

Patterns for building GraphQL APIs in Rails applications using graphql-ruby.

When This Skill Applies

• Designing GraphQL schemas and types
• Implementing resolvers and mutations
• Optimizing queries and preventing N+1 problems
• Handling authentication/authorization in GraphQL
• Implementing subscriptions for real-time updates
• Structuring the app/graphql directory

Quick Reference

Detailed Documentation

• [patterns.md](patterns.md) - Complete GraphQL patterns with examples

Type Definition Basics

# app/graphql/types/user_type.rb
module Types
  class UserType < Types::BaseObject
    field :id, ID, null: false
    field :email, String, null: false
    field :name, String
    field :created_at, GraphQL::Types::ISO8601DateTime, null: false

    field :posts, [Types::PostType], null: true
    field :posts_count, Integer, null: false

    def posts_count
      object.posts.size  # Use size for counter cache
    end
  end
end

Mutation Basics

# app/graphql/mutations/create_post.rb
module Mutations
  class CreatePost < BaseMutation
    argument :title, String, required: true
    argument :content, String, required: true

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

    def resolve(title:, content:)
      post = context[:current_user].posts.build(title: title, content: content)

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

N+1 Prevention with DataLoader

# app/graphql/sources/record_loader.rb
class Sources::RecordLoader < GraphQL::Dataloader::Source
  def initialize(model_class)
    @model_class = model_class
  end

  def fetch(ids)
    records = @model_class.where(id: ids).index_by(&:id)
    ids.map { |id| records[id] }
  end
end

# Usage in type
def author
  dataloader.with(Sources::RecordLoader, User).load(object.user_id)
end

Key Principles

• Type Safety: Define explicit types for all fields
• Null Handling: Be intentional about null: true/false
• Batch Loading: Use DataLoader to prevent N+1 queries
• Authorization: Check permissions at field level
• Error Handling: Return structured errors, not exceptions