Fix ActiveRecord locking column defaults not getting persisted

When partial inserts are enabled, overridden db defaults are ignored. This
results in locking columns having a nil value for new records if the db default
is null. This happens because the list of changed attributes for new records is
always assumed to be empty.

Solution: When a new record's default attributes are set, also initialize the
list of changed attributes by comparing current values against what's stored as
the column defaults in the database.

activerecord/lib/active_record/core.rb

@@ -176,6 +176,7 @@ def initialize(attributes = nil)
       @columns_hash = self.class.column_types.dup
 
       init_internals
+      init_changed_attributes
       ensure_proper_type
       populate_with_current_scope_attributes
 
@@ -246,9 +247,7 @@ def initialize_dup(other) # :nodoc:
       run_callbacks(:initialize) unless _initialize_callbacks.empty?
 
       @changed_attributes = {}
-      self.class.column_defaults.each do |attr, orig_value|
-        @changed_attributes[attr] = orig_value if _field_changed?(attr, orig_value, @attributes[attr])
-      end
+      init_changed_attributes
 
       @aggregation_cache = {}
       @association_cache = {}
@@ -433,5 +432,14 @@ def init_internals
       @transaction_state        = nil
       @reflects_state           = [false]
     end
+
+    def init_changed_attributes
+      # Intentionally avoid using #column_defaults since overriden defaults (as is done in
+      # optimistic locking) won't get written unless they get marked as changed
+      self.class.columns.each do |c|
+        attr, orig_value = c.name, c.default
+        @changed_attributes[attr] = orig_value if _field_changed?(attr, orig_value, @attributes[attr])
+      end
+    end
   end
 end

activerecord/test/cases/locking_test.rb

@@ -193,11 +193,19 @@ def test_lock_column_is_mass_assignable
   def test_lock_without_default_sets_version_to_zero
     t1 = LockWithoutDefault.new
     assert_equal 0, t1.lock_version
+
+    t1.save
+    t1 = LockWithoutDefault.find(t1.id)
+    assert_equal 0, t1.lock_version
   end
 
   def test_lock_with_custom_column_without_default_sets_version_to_zero
     t1 = LockWithCustomColumnWithoutDefault.new
     assert_equal 0, t1.custom_lock_version
+
+    t1.save
+    t1 = LockWithCustomColumnWithoutDefault.find(t1.id)
+    assert_equal 0, t1.custom_lock_version
   end
 
   def test_readonly_attributes