Match destroyed_by_association for has_one to has_many

When a has_many association is destroyed by `dependent: destroy`,
destroyed_by_association is set to the reflection, and this can be
checked in callbacks.  This matches that behaviour for has_one
associations.

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   When a `has_one` association is destroyed by `dependent: destroy`,
+    `destroyed_by_association` will now be set to the reflection, matching the
+    behaviour of `has_many` associations.
+
+    *Lisa Ugray*
+
 *   Fix `unscoped(where: [columns])` removing the wrong bind values
 
     When the `where` is called on a relation after a `or`, unscoping the column of that later `where` removed

activerecord/lib/active_record/associations/has_one_association.rb

@@ -56,6 +56,7 @@ module ActiveRecord
           when :delete
             target.delete
           when :destroy
+            target.destroyed_by_association = reflection
             target.destroy
           when :nullify
             target.update_columns(reflection.foreign_key => nil) if target.persisted?
@@ -78,6 +79,7 @@ module ActiveRecord
           when :delete
             target.delete
           when :destroy
+            target.destroyed_by_association = reflection
             target.destroy
           else
             nullify_owner_attributes(target)

activerecord/test/cases/associations/has_one_associations_test.rb

@@ -688,4 +688,38 @@ class HasOneAssociationsTest < ActiveRecord::TestCase
       SpecialAuthor.joins(book: :subscription).where.not(where_clause)
     end
   end
+
+  class DestroyByParentBook < ActiveRecord::Base
+    self.table_name = "books"
+    belongs_to :author, class_name: "DestroyByParentAuthor"
+    before_destroy :dont, unless: :destroyed_by_association
+
+    def dont
+      throw(:abort)
+    end
+  end
+
+  class DestroyByParentAuthor < ActiveRecord::Base
+    self.table_name = "authors"
+    has_one :book, class_name: "DestroyByParentBook", foreign_key: "author_id", dependent: :destroy
+  end
+
+  test "destroyed_by_association set in child destroy callback on parent destroy" do
+    author = DestroyByParentAuthor.create!(name: "Test")
+    book = DestroyByParentBook.create!(author: author)
+
+    author.destroy
+
+    assert_not DestroyByParentBook.exists?(book.id)
+  end
+
+  test "destroyed_by_association set in child destroy callback on replace" do
+    author = DestroyByParentAuthor.create!(name: "Test")
+    book = DestroyByParentBook.create!(author: author)
+
+    author.book = DestroyByParentBook.create!
+    author.save!
+
+    assert_not DestroyByParentBook.exists?(book.id)
+  end
 end