3 files changed, 17 insertions, 16 deletions
diff --git a/activemodel/lib/active_model/attribute_set.rb b/activemodel/lib/active_model/attribute_set.rb
index a892accbc6..54a5dd4064 100644
--- a/activemodel/lib/active_model/attribute_set.rb
+++ b/activemodel/lib/active_model/attribute_set.rb
@@ -5,7 +5,7 @@ require "active_model/attribute_set/yaml_encoder"
 
 module ActiveModel
   class AttributeSet # :nodoc:
-    delegate :each_value, :fetch, to: :attributes
+    delegate :each_value, :fetch, :except, to: :attributes
 
     def initialize(attributes)
       @attributes = attributes
diff --git a/activemodel/lib/active_model/attribute_set/builder.rb b/activemodel/lib/active_model/attribute_set/builder.rb
index f94f47370f..758eb830fc 100644
--- a/activemodel/lib/active_model/attribute_set/builder.rb
+++ b/activemodel/lib/active_model/attribute_set/builder.rb
@@ -5,35 +5,30 @@ require "active_model/attribute"
 module ActiveModel
   class AttributeSet # :nodoc:
     class Builder # :nodoc:
-      attr_reader :types, :always_initialized, :default
+      attr_reader :types, :default_attributes
 
-      def initialize(types, always_initialized = nil, &default)
+      def initialize(types, default_attributes = {})
         @types = types
-        @always_initialized = always_initialized
-        @default = default
+        @default_attributes = default_attributes
       end
 
       def build_from_database(values = {}, additional_types = {})
-        if always_initialized && !values.key?(always_initialized)
-          values[always_initialized] = nil
-        end
-
-        attributes = LazyAttributeHash.new(types, values, additional_types, &default)
+        attributes = LazyAttributeHash.new(types, values, additional_types, default_attributes)
         AttributeSet.new(attributes)
       end
     end
   end
 
   class LazyAttributeHash # :nodoc:
-    delegate :transform_values, :each_key, :each_value, :fetch, to: :materialize
+    delegate :transform_values, :each_key, :each_value, :fetch, :except, to: :materialize
 
-    def initialize(types, values, additional_types, &default)
+    def initialize(types, values, additional_types, default_attributes)
       @types = types
       @values = values
       @additional_types = additional_types
       @materialized = false
       @delegate_hash = {}
-      @default = default || proc {}
+      @default_attributes = default_attributes
     end
 
     def key?(key)
@@ -94,7 +89,7 @@ module ActiveModel
 
     protected
 
-      attr_reader :types, :values, :additional_types, :delegate_hash, :default
+      attr_reader :types, :values, :additional_types, :delegate_hash, :default_attributes
 
       def materialize
         unless @materialized
@@ -117,7 +112,12 @@ module ActiveModel
         if value_present
           delegate_hash[name] = Attribute.from_database(name, value, type)
         elsif types.key?(name)
-          delegate_hash[name] = default.call(name) || Attribute.uninitialized(name, type)
+          attr = default_attributes[name]
+          if attr
+            delegate_hash[name] = attr.dup
+          else
+            delegate_hash[name] = Attribute.uninitialized(name, type)
+          end
         end
       end
   end
diff --git a/activemodel/test/cases/attribute_set_test.rb b/activemodel/test/cases/attribute_set_test.rb
index 02c44c5d45..6e522d6c80 100644
--- a/activemodel/test/cases/attribute_set_test.rb
+++ b/activemodel/test/cases/attribute_set_test.rb
@@ -163,7 +163,8 @@ module ActiveModel
     end
 
     test "the primary_key is always initialized" do
-      builder = AttributeSet::Builder.new({ foo: Type::Integer.new }, :foo)
+      defaults = { foo: Attribute.from_user(:foo, nil, nil) }
+      builder = AttributeSet::Builder.new({ foo: Type::Integer.new }, defaults)
       attributes = builder.build_from_database
 
       assert attributes.key?(:foo)