Add accepts_nested_attributes_for support when using delegated_type (rails#41717)

Co-Authored-By: Zachary Scott <[email protected]>

Co-authored-by: Sjors Baltus <[email protected]>
Co-authored-by: Zachary Scott <[email protected]>

Author: 3 people

activerecord/CHANGELOG.md

@@ -1,3 +1,34 @@
+*   Add nested_attributes_for support for `delegated_type`
+
+    ```ruby
+    class Entry < ApplicationRecord
+      delegated_type :entryable, types: %w[ Message Comment ]
+      accepts_nested_attributes_for :entryable
+    end
+
+    entry = Entry.create(entryable_type: 'Message', entryable_attributes: { content: 'Hello world' })
+    # => #<Entry:0x00>
+    # id: 1
+    # entryable_id: 1,
+    # entryable_type: 'Message'
+    # ...>
+
+    entry.entryable
+    # => #<Message:0x01>
+    # id: 1
+    # content: 'Hello world'
+    # ...>
+    ```
+
+    Previously it would raise an error:
+
+    ```ruby
+    Entry.create(entryable_type: 'Message', entryable_attributes: { content: 'Hello world' })
+    # ArgumentError: Cannot build association `entryable'. Are you trying to build a polymorphic one-to-one association?
+    ```
+
+    *Sjors Baltus*
+
 *   Use subquery for DELETE with GROUP_BY and HAVING clauses.
 
     Prior to this change, deletes with GROUP_BY and HAVING were returning an error.

activerecord/lib/active_record/delegated_type.rb

@@ -137,6 +137,21 @@ module ActiveRecord
   #   end
   #
   # Now you can list a bunch of entries, call +Entry#title+, and polymorphism will provide you with the answer.
+  #
+  # == Nested Attributes
+  #
+  # Enabling nested attributes on a delegated_type association allows you to
+  # create the entry and message in one go:
+  #
+  #   class Entry < ApplicationRecord
+  #     delegated_type :entryable, types: %w[ Message Comment ]
+  #     accepts_nested_attributes_for :entryable
+  #   end
+  #
+  #   params = { entry: { entryable_type: 'Message', entryable_attributes: { subject: 'Smiling' } } }
+  #   entry = Entry.create(params[:entry])
+  #   entry.entryable.id # => 2
+  #   entry.entryable.subject # => 'Smiling'
   module DelegatedType
     # 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
@@ -207,6 +222,10 @@ def define_delegated_type_methods(role, types:, options:)
           public_send("#{role}_class").model_name.singular.inquiry
         end
 
+        define_method "build_#{role}" do |*params|
+          public_send("#{role}=", public_send("#{role}_class").new(*params))
+        end
+
         types.each do |type|
           scope_name = type.tableize.tr("/", "_")
           singular   = scope_name.singularize

activerecord/test/cases/delegated_type_test.rb

@@ -72,4 +72,9 @@ class DelegatedTypeTest < ActiveRecord::TestCase
     assert_equal @uuid_entry_with_comment.entryable_uuid, @uuid_entry_with_comment.uuid_comment_uuid
     assert_nil @uuid_entry_with_comment.uuid_message_uuid
   end
+
+  test "builder method" do
+    assert_respond_to Entry.new, :build_entryable
+    assert_equal Message, Entry.new(entryable_type: "Message").build_entryable.class
+  end
 end

activerecord/test/cases/nested_attributes_test.rb

@@ -11,6 +11,8 @@
 require "models/interest"
 require "models/owner"
 require "models/pet"
+require "models/entry"
+require "models/message"
 require "active_support/hash_with_indifferent_access"
 
 class TestNestedAttributesInGeneral < ActiveRecord::TestCase
@@ -1117,3 +1119,15 @@ def test_extend_affects_nested_attributes
     assert_equal "from extension", pirate.treasures[0].name
   end
 end
+
+class TestNestedAttributesForDelegatedType < ActiveRecord::TestCase
+  setup do
+    Entry.accepts_nested_attributes_for :entryable
+    @entry = Entry.new(entryable_type: "Message", entryable_attributes: { subject: "Hello world!" })
+  end
+
+  def test_should_build_a_new_record_based_on_the_delegated_type
+    assert_not_predicate @entry.entryable, :persisted?
+    assert_equal "Hello world!", @entry.entryable.subject
+  end
+end