Add tests around has_one changing the target and validators

Ref: rails#48330

When replacing the has_one target, it seems more correct to first
delete the old record, so that if the associated model has a uniqueness
validator, it won't fail.

Both the delete and the insert are in a single transaction, so if the new
record fail to be saved, the transaction will be rolled back, which seems
correct.

If `dependent: :destroy` isn't set, Active Record will try to orphan the
record, which may or may not be valid depending on the schema and validators.

Co-Authored-By: zzak <[email protected]>

Author: byroot

activerecord/test/cases/associations/has_one_associations_test.rb

@@ -20,6 +20,8 @@
 require "models/membership"
 require "models/parrot"
 require "models/cpk"
+require "models/user"
+require "models/content"
 
 class HasOneAssociationsTest < ActiveRecord::TestCase
   self.use_transactional_tests = false unless supports_savepoints?
@@ -905,6 +907,46 @@ def test_has_one_with_touch_option_on_nonpersisted_built_associations_doesnt_upd
     end
   end
 
+  class InvalidPost < Post
+    validate :always_fail
+
+    def always_fail
+      errors.add(:base, "Oops")
+    end
+  end
+
+  test "preserve old record if new one is invalid" do
+    author = Author.create!(id: 33, name: "Hank Moody")
+    author.create_post!(id: 1234, title: "Some title", body: "Some content")
+
+    assert_no_difference -> { Post.count } do
+      assert_raises ActiveRecord::RecordNotSaved do
+        author.post = InvalidPost.new(title: "Some title", body: "Some content")
+      end
+      author.save!
+    end
+  end
+
+  class SpecialContentPosition < ActiveRecord::Base
+    self.table_name = "content_positions"
+    belongs_to :content, class_name: name + "::SpecialContentPosition"
+    validates :content_id, presence: true, uniqueness: true
+  end
+
+  class SpecialContent < ActiveRecord::Base
+    self.table_name = "content"
+    has_one :content_position, dependent: :destroy, foreign_key: :content_id, class_name: SpecialContentPosition.name
+  end
+
+  test "uniqueness validator doesn't prevent from replacing the old record if dependent: :destroy is set" do
+    content = SpecialContent.create!
+    content.create_content_position!
+
+    assert_no_difference -> { ContentPosition.count } do
+      content.create_content_position!
+    end
+  end
+
   test "composite primary key malformed association" do
     error = assert_raises(ActiveRecord::CompositePrimaryKeyMismatchError) do
       order = Cpk::BrokenOrder.new(id: [1, 2], book: Cpk::Book.new(title: "Some book"))