Clean up the implementation of AR::Dirty

This moves a bit more of the logic required for dirty checking into the
attribute objects. I had hoped to remove the `with_value_from_database`
stuff, but unfortunately just calling `dup` on the attribute objects
isn't enough, since the values might contain deeply nested data
structures. I think this can be cleaned up further.

This makes most dirty checking become lazy, and reduces the number of
object allocations and amount of CPU time when assigning a value. This
opens the door (but doesn't quite finish) to improving the performance
of writes to a place comparable to 4.1

2 files changed, 33 insertions, 0 deletions

diff --git a/activerecord/test/cases/attribute_set_test.rb b/activerecord/test/cases/attribute_set_test.rb
index 9d927481ec..7524243270 100644
--- a/activerecord/test/cases/attribute_set_test.rb
+++ b/activerecord/test/cases/attribute_set_test.rb
@@ -160,6 +160,9 @@ module ActiveRecord
         return if value.nil?
         value + " from database"
       end
+
+      def assert_valid_value(*)
+      end
     end
 
     test "write_from_database sets the attribute with database typecasting" do
@@ -207,5 +210,16 @@ module ActiveRecord
 
       assert_equal [:foo], attributes.accessed
     end
+
+    test "#map returns a new attribute set with the changes applied" do
+      builder = AttributeSet::Builder.new(foo: Type::Integer.new, bar: Type::Integer.new)
+      attributes = builder.build_from_database(foo: "1", bar: "2")
+      new_attributes = attributes.map do |attr|
+        attr.with_cast_value(attr.value + 1)
+      end
+
+      assert_equal 2, new_attributes.fetch_value(:foo)
+      assert_equal 3, new_attributes.fetch_value(:bar)
+    end
   end
 end
diff --git a/activerecord/test/cases/attribute_test.rb b/activerecord/test/cases/attribute_test.rb
index c69782d93f..0f216b7a13 100644
--- a/activerecord/test/cases/attribute_test.rb
+++ b/activerecord/test/cases/attribute_test.rb
@@ -107,6 +107,9 @@ module ActiveRecord
       def deserialize(value)
         value + " from database"
       end
+
+      def assert_valid_value(*)
+      end
     end
 
     test "with_value_from_user returns a new attribute with the value from the user" do
@@ -186,5 +189,21 @@ module ActiveRecord
 
       assert_not attribute.changed_in_place_from?("bar")
     end
+
+    test "with_value_from_user validates the value" do
+      type = Type::Value.new
+      type.define_singleton_method(:assert_valid_value) do |value|
+        if value == 1
+          raise ArgumentError
+        end
+      end
+
+      attribute = Attribute.from_database(:foo, 1, type)
+      assert_equal 1, attribute.value
+      assert_equal 2, attribute.with_value_from_user(2).value
+      assert_raises ArgumentError do
+        attribute.with_value_from_user(1)
+      end
+    end
   end
 end