Merge pull request #18155 from bogdan/collection_association_double_element_fix

Bugfix collection association #create method

activerecord/lib/active_record/associations/collection_association.rb

@@ -414,12 +414,16 @@ def add_to_target(record, skip_callbacks = false, &block)
 
       def replace_on_target(record, index, skip_callbacks)
         callback(:before_add, record) unless skip_callbacks
+
+        was_loaded = loaded?
         yield(record) if block_given?
 
-        if index
-          @target[index] = record
-        else
-          @target << record
+        unless !was_loaded && loaded?
+          if index
+            @target[index] = record
+          else
+            @target << record
+          end
         end
 
         callback(:after_add, record) unless skip_callbacks

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -2348,6 +2348,12 @@ def self.name
     assert_equal [first_bulb, second_bulb], car.bulbs
   end
 
+  test 'double insertion of new object to association when same association used in the after create callback of a new object' do
+    car = Car.create!
+    car.bulbs << TrickyBulb.new
+    assert_equal 1, car.bulbs.size
+  end
+
   def test_association_force_reload_with_only_true_is_deprecated
     company = Company.find(1)
 

activerecord/test/models/bulb.rb

@@ -50,3 +50,9 @@ class FailedBulb < Bulb
     throw(:abort)
   end
 end
+
+class TrickyBulb < Bulb
+  after_create do |record|
+    record.car.bulbs.to_a
+  end
+end