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 42dd5d9f2976677a4bf22347f2dde1a8135dfbb4 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.

1 files changed, 16 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/has_one_associations_test.rb b/activerecord/test/cases/associations/has_one_associations_test.rb
index 4ed09a3bf7..0e48fbca9c 100644
--- a/activerecord/test/cases/associations/has_one_associations_test.rb
+++ b/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