3 files changed, 26 insertions, 4 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 9f64941065..5f31ca830c 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Fix bug where has_one associaton record update result in crash, when replaced with itself.
+
+    Fixes #12834.
+
+    *Denis Redozubov*, *Sergio Cambra*
+
 *   Log bind variables after they are type casted. This makes it more
     transparent what values are actually sent to the database.
 
diff --git a/activerecord/lib/active_record/associations/has_one_association.rb b/activerecord/lib/active_record/associations/has_one_association.rb
index 0008600418..944caacab6 100644
--- a/activerecord/lib/active_record/associations/has_one_association.rb
+++ b/activerecord/lib/active_record/associations/has_one_association.rb
@@ -26,11 +26,13 @@ module ActiveRecord
         load_target
 
         return self.target if !(target || record)
-        if (target != record) || record.changed?
+
+        assigning_another_record = target != record
+        if assigning_another_record || record.changed?
           save &&= owner.persisted?
 
           transaction_if(save) do
-            remove_target!(options[:dependent]) if target && !target.destroyed?
+            remove_target!(options[:dependent]) if target && !target.destroyed? && assigning_another_record
 
             if record
               set_owner_attributes(record)
diff --git a/activerecord/test/cases/associations/has_one_associations_test.rb b/activerecord/test/cases/associations/has_one_associations_test.rb
index cdd386187b..a7a8e0c5c6 100644
--- a/activerecord/test/cases/associations/has_one_associations_test.rb
+++ b/activerecord/test/cases/associations/has_one_associations_test.rb
@@ -509,16 +509,30 @@ class HasOneAssociationsTest < ActiveRecord::TestCase
     assert_no_queries { Firm.new.account = account }
   end
 
-  def test_has_one_assignment_triggers_save_on_change
+  def test_has_one_assignment_dont_triggers_save_on_change_of_same_object
     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(1) do
+      # One query for updating name, not triggering query for updating pirate_id
+      pirate.ship = ship
+    end
+
+    assert_equal 'new name', pirate.ship.reload.name
+  end
+
+  def test_has_one_assignment_triggers_save_on_change_on_replacing_object
+    pirate = Pirate.create!(catchphrase: "Don' botharrr talkin' like one, savvy?")
+    ship = pirate.build_ship(name: 'old name')
+    ship.save!
+
+    new_ship = Ship.create(name: 'new name')
     assert_queries(2) do
       # One query for updating name and second query for updating pirate_id
-      pirate.ship = ship
+      pirate.ship = new_ship
     end
 
     assert_equal 'new name', pirate.ship.reload.name