destroys association records before saving/inserting new association records

fixes bug introduced by  #3329
These are the conditions necessary to reproduce the bug:
- For an association, autosave => true.
- An association record is being destroyed
- A new association record is being created.
- There is a unique index one of the association's fields.
- The record being created has the same value as the record being
destroyed on the indexed field.

Before, the deletion of records was postponed until after all
insertions/saves.  Therefore the new record with the identical value in
the indexed field caused a non-unique value error to be thrown at the database
level.

With this fix, the deletions happen first, before the insertions/saves.
Therefore the record with the duplicate value is gone from the database
before the new record is created, thereby avoiding the non-uniuqe value
error.

1 files changed, 14 insertions, 0 deletions

diff --git a/activerecord/test/cases/autosave_association_test.rb b/activerecord/test/cases/autosave_association_test.rb
index 536ff4882c..552bbd3f52 100644
--- a/activerecord/test/cases/autosave_association_test.rb
+++ b/activerecord/test/cases/autosave_association_test.rb
@@ -764,6 +764,20 @@ class TestDestroyAsPartOfAutosaveAssociation < ActiveRecord::TestCase
     assert_equal 2, @pirate.birds.reload.length
   end
 
+  def test_should_save_new_record_that_has_same_value_as_existing_record_marked_for_destruction_on_field_that_has_unique_index
+    Bird.connection.add_index :birds, :name, unique: true
+
+    3.times { |i| @pirate.birds.create(name: "birds_#{i}") }
+
+    @pirate.birds[0].mark_for_destruction
+    @pirate.birds.build(name: @pirate.birds[0].name)
+    @pirate.save!
+
+    assert_equal 3, @pirate.birds.reload.length
+  ensure
+    Bird.connection.remove_index :birds, column: :name
+  end
+
   # Add and remove callbacks tests for association collections.
   %w{ method proc }.each do |callback_type|
     define_method("test_should_run_add_callback_#{callback_type}s_for_has_many") do