Rails GraphQL Patterns

Analyze and recommend patterns for building GraphQL APIs in Rails with graphql-ruby.

Follow standard graphql-ruby conventions for type definitions, input types, enum types, query types, and basic mutations. This skill focuses on non-obvious patterns and opinionated decisions.

Core Principles

1. Batch loading: Always use DataLoader to prevent N+1 queries — never resolve associations directly
2. Structured errors: Return error arrays in mutation payloads, not exceptions
3. Field-level authorization: Check permissions per field, not just per query
4. Complexity limits: Set max_complexity and max_depth on the schema
5. Null annotations: Be intentional with null: true/false on every field

DataLoader for N+1 Prevention

Every association field MUST use DataLoader. Never call object.association directly in a resolver.

class Sources::RecordLoader < GraphQL::Dataloader::Source
  def initialize(model_class, column: :id)
    @model_class = model_class
    @column = column
  end

  def fetch(ids)
    records = @model_class.where(@column => ids).index_by { |r| r.send(@column) }
    ids.map { |id| records[id] }
  end
end

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

See patterns.md for AssociationLoader and CountLoader variants.

Connection Types (Pagination)

Always use connection types for list fields — never return unbounded arrays.

module Types
  class PostConnectionType < GraphQL::Types::Connection
    edge_type(Types::PostEdgeType)
    field :total_count, Integer, null: false

    def total_count
      object.items.size
    end
  end
end

# In query type
field :posts, Types::PostConnectionType, null: false, connection: true do
  argument :filter, Types::PostFilterInput, required: false
end

Subscriptions — Triggering from Models

# Trigger from model after_create_commit, not from mutations
class Post < ApplicationRecord
  after_create_commit :notify_subscribers

  private

  def notify_subscribers
    MyAppSchema.subscriptions.trigger(:post_created, {}, self)
  end
end

Schema Configuration

class MyAppSchema < GraphQL::Schema
  max_complexity 300
  max_depth 15
  default_max_page_size 25

  use GraphQL::Dataloader

  rescue_from ActiveRecord::RecordNotFound do |_err, _obj, _args, _ctx, field|
    raise GraphQL::ExecutionError, "#{field.type.unwrap.graphql_name} not found"
  end
end

Per-field complexity for expensive operations:

field :expensive_field, String do
  complexity 50
end

Anti-Patterns

Output Format

When analyzing or creating GraphQL components, provide:

1. Type/mutation file with proper null annotations
2. DataLoader source if associations are resolved
3. Schema configuration (complexity, depth, pagination)
4. Test outline executing queries against the schema
5. Authorization strategy (context, field-level checks)

Error Handling

• Mutations return { resource: nil, errors: [...] } on validation failure
• Use rescue_from in schema for RecordNotFound mapped to GraphQL::ExecutionError
• Field-level: return nil or raise GraphQL::ExecutionError for unauthorized access
• Never expose internal error details to clients in production