delegated_type

Defines this as a class that’ll delegate its type for the passed role to the class references in types. That’ll create a polymorphic belongs_to relationship to that role, and it’ll add all the delegated type convenience methods:

class Entry < ApplicationRecord
  delegated_type :entryable, types: %w[ Message Comment ], dependent: :destroy
end

Entry#entryable_class # => +Message+ or +Comment+
Entry#entryable_name  # => "message" or "comment"
Entry.messages        # => Entry.where(entryable_type: "Message")
Entry#message?        # => true when entryable_type == "Message"
Entry#message         # => returns the message record, when entryable_type == "Message", otherwise nil
Entry#message_id      # => returns entryable_id, when entryable_type == "Message", otherwise nil
Entry.comments        # => Entry.where(entryable_type: "Comment")
Entry#comment?        # => true when entryable_type == "Comment"
Entry#comment         # => returns the comment record, when entryable_type == "Comment", otherwise nil
Entry#comment_id      # => returns entryable_id, when entryable_type == "Comment", otherwise nil

You can also declare namespaced types:

class Entry < ApplicationRecord
  delegated_type :entryable, types: %w[ Message Comment Access::NoticeMessage ], dependent: :destroy
end

Entry.access_notice_messages
entry.access_notice_message
entry.access_notice_message?

Options

The options are passed directly to the belongs_to call, so this is where you declare dependent etc. The following options can be included to specialize the behavior of the delegated type convenience methods.

:foreign_key

Specify the foreign key used for the convenience methods. By default this is guessed to be the passed role with an “_id” suffix. So a class that defines a delegated_type :entryable, types: %w[ Message Comment ] association will use “entryable_id” as the default :foreign_key.

:foreign_type

Specify the column used to store the associated object’s type. By default this is inferred to be the passed role with a “_type” suffix. A class that defines a delegated_type :entryable, types: %w[ Message Comment ] association will use “entryable_type” as the default :foreign_type.

:primary_key

Specify the method that returns the primary key of associated object used for the convenience methods. By default this is id.

Option examples:

class Entry < ApplicationRecord
  delegated_type :entryable, types: %w[ Message Comment ], primary_key: :uuid, foreign_key: :entryable_uuid
end

Entry#message_uuid      # => returns entryable_uuid, when entryable_type == "Message", otherwise nil
Entry#comment_uuid      # => returns entryable_uuid, when entryable_type == "Comment", otherwise nil

# File activerecord/lib/active_record/delegated_type.rb, line 211
    def delegated_type(role, types,, **options)
      belongs_to role, options.delete(:scope), **options.merge(polymorphic: true)
      define_delegated_type_methods role, types: types, options: options
    end