Fix #8856 Ensure has_one association=(associate) triggers save.

activerecord/lib/active_record/associations.rb states:

    # [association=(associate)]
    #   Assigns the associate object, extracts the primary key, sets it as the foreign key,
    #   and saves the associate object.

Since commit 42dd5d9f29 to fix #7191, this
is no longer the case if the associate has changed, but is the same
object. For example:

    # Pirate has_one :ship
    pirate = Pirate.create!(catchphrase: "A Pirate")
    ship = pirate.build_ship(name: 'old name')
    ship.save!

    ship.name = 'new name'
    pirate.ship = ship

That last line should trigger a save. Although we are not changing the
association, the associate (ship) has changed.

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Trigger a save on `has_one association=(associate)` when the associate contents have changed.
+
+    Fix #8856.
+
+    *Chris Thompson*
+
 *   Abort a rake task when missing db/structure.sql like `db:schema:load` task.
 
     *kennyj*

activerecord/lib/active_record/associations/has_one_association.rb

@@ -25,9 +25,8 @@ module ActiveRecord
         raise_on_type_mismatch!(record) if record
         load_target
 
-        # If target and record are nil, or target is equal to record,
-        # we don't need to have transaction.
-        if (target || record) && target != record
+        return self.target if !(target || record)
+        if (target != record) || record.changed?
           transaction_if(save) do
             remove_target!(options[:dependent]) if target && !target.destroyed?
 

activerecord/test/cases/associations/has_one_associations_test.rb

@@ -522,4 +522,20 @@ class HasOneAssociationsTest < ActiveRecord::TestCase
     account = Account.find(2)
     assert_queries { company.account = account }
   end
+
+  def test_has_one_assignment_triggers_save_on_change
+    pirate = Pirate.create!(catchphrase: "Don' botharrr talkin' like one, savvy?")
+    ship = pirate.build_ship(name: 'old name')
+    ship.save!
+
+    ship.name = 'new name'
+    assert ship.changed?
+    assert_queries(2) do
+      # One query for updating name and second query for updating pirate_id
+      pirate.ship = ship
+    end
+
+    assert_equal 'new name', pirate.ship.reload.name
+  end
+
 end