Return a null object from `AttributeSet#[]`

activerecord/lib/active_record/attribute_methods/before_type_cast.rb

@@ -43,9 +43,7 @@ module ActiveRecord
       #   task.read_attribute_before_type_cast('completed_on') # => "2012-10-21"
       #   task.read_attribute_before_type_cast(:completed_on)  # => "2012-10-21"
       def read_attribute_before_type_cast(attr_name)
-        if attr = @attributes[attr_name.to_s]
-          attr.value_before_type_cast
-        end
+        @attributes[attr_name.to_s].value_before_type_cast
       end
 
       # Returns a hash of attributes before typecasting and deserialization.

activerecord/lib/active_record/attribute_methods/dirty.rb

@@ -129,7 +129,7 @@ module ActiveRecord
       end
 
       def _field_changed?(attr, old_value)
-        attribute_named(attr).changed_from?(old_value)
+        @attributes[attr].changed_from?(old_value)
       end
 
       def changed_in_place
@@ -140,7 +140,7 @@ module ActiveRecord
 
       def changed_in_place?(attr_name)
         old_value = original_raw_attribute(attr_name)
-        attribute_named(attr_name).changed_in_place_from?(old_value)
+        @attributes[attr_name].changed_in_place_from?(old_value)
       end
 
       def original_raw_attribute(attr_name)
@@ -154,7 +154,7 @@ module ActiveRecord
       end
 
       def store_original_raw_attribute(attr_name)
-        original_raw_attributes[attr_name] = attribute_named(attr_name).value_for_database
+        original_raw_attributes[attr_name] = @attributes[attr_name].value_for_database
       end
 
       def store_original_raw_attributes

activerecord/lib/active_record/attribute_methods/read.rb

@@ -99,10 +99,6 @@ module ActiveRecord
       def attribute(attribute_name)
         read_attribute(attribute_name)
       end
-
-      def attribute_named(attribute_name)
-        @attributes.fetch(attribute_name, Attribute::Null)
-      end
     end
   end
 end

activerecord/lib/active_record/attribute_set.rb

@@ -35,7 +35,8 @@ module ActiveRecord
       end
 
       def build_from_database(values, additional_types = {})
-        attributes = values.each_with_object({}) do |(name, value), hash|
+        attributes = Hash.new(Attribute::Null)
+        values.each_with_object(attributes) do |(name, value), hash|
           type = additional_types.fetch(name, @types[name])
           hash[name] = Attribute.from_database(value, type)
         end

activerecord/lib/active_record/core.rb

@@ -523,7 +523,9 @@ module ActiveRecord
 
     def init_internals
       pk = self.class.primary_key
-      @attributes[pk] ||= Attribute.from_database(nil, type_for_attribute(pk))
+      unless @attributes.include?(pk)
+        @attributes[pk] = Attribute.from_database(nil, type_for_attribute(pk))
+      end
 
       @aggregation_cache        = {}
       @association_cache        = {}

activerecord/test/cases/attribute_set_test.rb

@@ -18,6 +18,14 @@ module ActiveRecord
       assert_equal 4, attributes[:bar].value
     end
 
+    test "[] returns a null object" do
+      builder = AttributeSet::Builder.new(foo: Type::Float.new)
+      attributes = builder.build_from_database(foo: '3.3')
+
+      assert_equal '3.3', attributes[:foo].value_before_type_cast
+      assert_equal nil, attributes[:bar].value_before_type_cast
+    end
+
     test "duping creates a new hash and dups each attribute" do
       builder = AttributeSet::Builder.new(foo: Type::Integer.new, bar: Type::String.new)
       attributes = builder.build_from_database(foo: 1, bar: 'foo')