Persist user provided default values, even if unchanged

This is a usability change to fix a quirk from our definition of partial
writes. By default, we only persist changed attributes. When creating a
new record, this is assumed that the default values came from the
database. However, if the user provided a default, it will not be
persisted, since we didn't see it as "changed". Since this is a very
specific case, I wanted to isolate it with the other quirks that come
from user provided default values. The number of edge cases which are
presenting themselves are starting to make me wonder if we should just
remove the ability to assign a default, in favor of overriding
`initialize`. For the time being, this is required for the attributes
API to not have confusing behavior.

We had to delete one test, since this actually changes the meaning of
`.changed?` on Active Record models. It now specifically means
`changed_from_database?`. While I think this will make the attributes
API more ergonomic to use, it is a subtle change in definition (though
not a backwards incompatible one). We should probably figure out the
right place to document this. (Feel free to open a PR doing that if
you're reading this).

/cc @rafaelfranca @kirs @senny

This is an alternate implementation of #19921.

Close #19921.

[Sean Griffin & Kir Shatrov]

activerecord/lib/active_record/attribute/user_provided_default.rb

@@ -3,8 +3,9 @@
 module ActiveRecord
   class Attribute # :nodoc:
     class UserProvidedDefault < FromUser
-      def initialize(name, value, type)
+      def initialize(name, value, type, database_default)
         super(name, value, type)
+        @database_default = database_default
       end
 
       def type_cast(value)
@@ -14,6 +15,18 @@ def type_cast(value)
           super
         end
       end
+
+      def changed_in_place_from?(old_value)
+        super || changed_from?(database_default.value)
+      end
+
+      def with_type(type)
+        self.class.new(name, value_before_type_cast, type, database_default)
+      end
+
+      protected
+
+      attr_reader :database_default
     end
   end
 end

activerecord/lib/active_record/attributes.rb

@@ -242,6 +242,7 @@ def define_default_attribute(name, value, type, from_user:)
             name,
             value,
             type,
+            _default_attributes[name],
           )
         else
           default_attribute = Attribute.from_database(name, value, type)

activerecord/test/cases/attributes_test.rb

@@ -135,6 +135,12 @@ def deserialize(*)
       assert_equal 2, klass.new.counter
     end
 
+    test "user provided defaults are persisted even if unchanged" do
+      model = OverloadedType.create!
+
+      assert_equal "the overloaded default", model.reload.string_with_default
+    end
+
     if current_adapter?(:PostgreSQLAdapter)
       test "arrays types can be specified" do
         klass = Class.new(OverloadedType) do

activerecord/test/cases/dirty_test.rb

@@ -623,32 +623,6 @@ def test_datetime_attribute_doesnt_change_if_zone_is_modified_in_string
     end
   end
 
-  test "defaults with type that implements `serialize`" do
-    type = Class.new(ActiveRecord::Type::Value) do
-      def cast(value)
-        value.to_i
-      end
-
-      def serialize(value)
-        value.to_s
-      end
-    end
-
-    model_class = Class.new(ActiveRecord::Base) do
-      self.table_name = 'numeric_data'
-      attribute :foo, type.new, default: 1
-    end
-
-    model = model_class.new
-    assert_not model.foo_changed?
-
-    model = model_class.new(foo: 1)
-    assert_not model.foo_changed?
-
-    model = model_class.new(foo: '1')
-    assert_not model.foo_changed?
-  end
-
   test "in place mutation detection" do
     pirate = Pirate.create!(catchphrase: "arrrr")
     pirate.catchphrase << " matey!"