Custom Directives

Learn how to create custom directives with events, filters, and lifecycle hooks for advanced GraphQL behavior.

Overview

Directives in Rails GraphQL allow you to modify the behavior of fields, types, and other schema elements. Custom directives work as event listeners and triggers, enabling powerful runtime modifications to your GraphQL operations.

Basic Directive Structure

Create a custom directive by inheriting from Rails::GraphQL::Directive:

module GraphQL
  class MyDirective < Rails::GraphQL::Directive
    placed_on :field_definition
    
    desc 'Custom directive description'
    
    argument :my_arg, :string, null: false
  end
end

Directive Locations

Directives can be placed on specific schema locations using placed_on:

Execution Locations

• :query - Query operations
• :mutation - Mutation operations
• :subscription - Subscription operations
• :field - Field selections
• :fragment_definition - Fragment definitions
• :fragment_spread - Fragment spreads
• :inline_fragment - Inline fragments

Definition Locations

• :schema - Schema definition
• :scalar - Scalar types
• :object - Object types
• :field_definition - Field definitions
• :argument_definition - Argument definitions
• :interface - Interface types
• :union - Union types
• :enum - Enum types
• :enum_value - Enum values
• :input_object - Input object types
• :input_field_definition - Input field definitions

class MyDirective < Rails::GraphQL::Directive
  placed_on :field_definition, :argument_definition
end

Directive Arguments

Define arguments for your directive using the argument method:

class CachedDirective < Rails::GraphQL::Directive
  placed_on :query
  
  argument :id, :ID, null: false, desc: 'The unique identifier of the cached operation.'
  argument :ttl, :int, default: 3600, desc: 'Time to live in seconds'
end

Event Listeners

Directives can listen to lifecycle events using the on method:

Available Events

• :attach - When the directive is attached to an element
• :authorize - During authorization checks
• :organized - After field organization
• :prepared - After preparation phase
• :finalize - During finalization
• :query - Query execution
• :mutation - Mutation execution
• :subscription - Subscription execution

Basic Event Handler

class SkipDirective < Rails::GraphQL::Directive
  placed_on :field, :fragment_spread, :inline_fragment
  
  argument :if, :boolean, null: false
  
  on(:attach) do |source|
    source.skip! if args[:if]
  end
end

Event Filters

Use event filters to conditionally execute directive logic:

The for Filter

Target specific types:

class DeprecatedDirective < Rails::GraphQL::Directive
  placed_on :field_definition, :enum_value
  
  argument :reason, :string
  
  on(:finalize, for: Type::Enum) do |event|
    return unless event.current_value.deprecated?
    
    value = event.current_value.to_s
    item = "#{value} value for the #{event.source.gql_name} field"
    event.request.report_error(build_message(item))
  end
  
  private
  
  def build_message(item)
    result = "The #{item} is deprecated"
    result << ", reason: #{args.reason}" if args.reason.present?
    result << '.'
  end
end

The on Filter

Filter by the event target:

on(:organized, on: SomeType) do |event|
  # Only runs when the event is on SomeType
end

The during Filter

Filter by execution phase:

on(:prepared, during: :execution) do |event|
  # Only runs during execution phase
end

Real-World Examples

Rate Limiting Directive

module GraphQL
  class RateLimitDirective < Rails::GraphQL::Directive
    placed_on :field_definition
    
    desc 'Limits the rate at which a field can be accessed'
    
    argument :max_calls, :int, null: false, desc: 'Maximum calls per window'
    argument :window, :int, default: 60, desc: 'Time window in seconds'
    
    on(:organized) do |event|
      key = "rate_limit:#{event.request.context[:user_id]}:#{event.field.gql_name}"
      count = Rails.cache.increment(key, 1, expires_in: args.window.seconds)
      
      if count > args.max_calls
        event.request.report_error(
          "Rate limit exceeded. Max #{args.max_calls} calls per #{args.window} seconds."
        )
      end
    end
  end
end

Authentication Directive

module GraphQL
  class AuthDirective < Rails::GraphQL::Directive
    placed_on :field_definition, :object
    
    desc 'Requires authentication to access the field'
    
    argument :role, :string, desc: 'Required user role'
    
    on(:authorize) do |event|
      user = event.request.context[:current_user]
      
      unless user.present?
        raise GraphQL::ExecutionError, 'Authentication required'
      end
      
      if args.role.present? && !user.has_role?(args.role)
        raise GraphQL::ExecutionError, "Required role: #{args.role}"
      end
    end
  end
end

Logging Directive

module GraphQL
  class LogDirective < Rails::GraphQL::Directive
    placed_on :field_definition
    
    desc 'Logs field access and execution time'
    
    argument :level, :string, default: 'info'
    
    on(:organized) do |event|
      start_time = Time.current
      @start_time = start_time
      
      Rails.logger.public_send(
        args.level,
        "[GraphQL] Accessing field: #{event.field.gql_name}"
      )
    end
    
    on(:finalize) do |event|
      duration = ((Time.current - @start_time) * 1000).round(2)
      
      Rails.logger.public_send(
        args.level,
        "[GraphQL] Field #{event.field.gql_name} completed in #{duration}ms"
      )
    end
  end
end

Repeatable Directives

Mark a directive as repeatable to allow multiple instances:

class ValidateDirective < Rails::GraphQL::Directive
  self.repeatable = true
  
  placed_on :argument_definition, :input_field_definition
  
  argument :format, :string, null: false
  argument :message, :string
end

Usage:

argument :email, :string, directives: [
  ValidateDirective(format: 'email'),
  ValidateDirective(format: 'lowercase')
]

Using Directives

On Fields

field :deprecated_field, :string, directives: DeprecatedDirective(reason: 'Use newField instead')

On Enum Values

enum :status do
  add :pending
  add :active
  add :old_status, directives: DeprecatedDirective(reason: 'Use active instead')
end

Shortcut Syntax

# These are equivalent:
field :name, :string, directives: MyDirective(arg: 'value')
field :name, :string, directives: MyDirective.new(arg: 'value')

Accessing Directive Data

Inside directive event handlers:

on(:organized) do |event|
  # Access directive arguments
  args.my_argument
  
  # Access the field
  event.field.gql_name
  
  # Access the request
  event.request.context[:current_user]
  
  # Access the source type
  event.source.gql_name
  
  # Report errors
  event.request.report_error('Error message')
end

Best Practices

Keep Directives Focused

Each directive should have a single, well-defined purpose. Don’t create directives that do too many things.

Use Appropriate Event Listeners

Choose the right lifecycle event for your directive logic. Use :authorize for authentication, :organized for validation, and :finalize for cleanup.

Handle Errors Gracefully

Always provide clear error messages when directive validation fails. Use event.request.report_error for user-facing errors.

Document Your Directives

Use the desc method to provide clear documentation about what your directive does and how to use it.

Next Steps

Custom Scalars

Learn how to create custom scalar types

Advanced Types

Explore advanced type system features