Correct the behavior of virtual attributes on models loaded from the db

Previously we had primarily tested the behavior of these attributes by
calling `.new`, allowing this to slip through the cracks. There were a
few ways in which they were behaving incorrectly.

The biggest issue was that attempting to read the attribute would
through a `MissingAttribute` error. We've corrected this by returning
the default value when the attribute isn't backed by a database column.
This is super special cased, but I don't see a way to avoid this
conditional. I had considered handling this higher up in
`define_default_attribute`, but we don't have the relevant information
there as users can provide new defaults for database columns as well.

Once I corrected this, I had noticed that the attributes were always
being marked as changed. This is because the behavior of
`define_default_attribute` was treating them as assigned from
`Attribute::Null`.

Finally, with our new implementation, `LazyAttributeHash` could no
longer be marshalled, as it holds onto a proc. This has been corrected
as well. I've not handled YAML in that class, as we do additional work
higher up to avoid YAML dumping it at all.

Fixes #25787
Close #25841

activerecord/lib/active_record/attribute.rb

@@ -152,7 +152,7 @@ def original_value_for_database
     end
 
     def _original_value_for_database
-      value_for_database
+      type.serialize(original_value)
     end
 
     class FromDatabase < Attribute # :nodoc:
@@ -180,7 +180,7 @@ def type_cast(value)
         value
       end
 
-      def changed_in_place_from?(old_value)
+      def changed_in_place?
         false
       end
     end

activerecord/lib/active_record/attribute_set.rb

@@ -3,7 +3,7 @@
 
 module ActiveRecord
   class AttributeSet # :nodoc:
-    delegate :each_value, to: :attributes
+    delegate :each_value, :fetch, to: :attributes
 
     def initialize(attributes)
       @attributes = attributes

activerecord/lib/active_record/attribute_set/builder.rb

@@ -3,11 +3,12 @@
 module ActiveRecord
   class AttributeSet # :nodoc:
     class Builder # :nodoc:
-      attr_reader :types, :always_initialized
+      attr_reader :types, :always_initialized, :default
 
-      def initialize(types, always_initialized = nil)
+      def initialize(types, always_initialized = nil, &default)
         @types = types
         @always_initialized = always_initialized
+        @default = default
       end
 
       def build_from_database(values = {}, additional_types = {})
@@ -15,21 +16,22 @@ def build_from_database(values = {}, additional_types = {})
           values[always_initialized] = nil
         end
 
-        attributes = LazyAttributeHash.new(types, values, additional_types)
+        attributes = LazyAttributeHash.new(types, values, additional_types, &default)
         AttributeSet.new(attributes)
       end
     end
   end
 
   class LazyAttributeHash # :nodoc:
-    delegate :transform_values, :each_key, :each_value, to: :materialize
+    delegate :transform_values, :each_key, :each_value, :fetch, to: :materialize
 
-    def initialize(types, values, additional_types)
+    def initialize(types, values, additional_types, &default)
       @types = types
       @values = values
       @additional_types = additional_types
       @materialized = false
       @delegate_hash = {}
+      @default = default || proc {}
     end
 
     def key?(key)
@@ -76,9 +78,21 @@ def ==(other)
       end
     end
 
+    def marshal_dump
+      materialize
+    end
+
+    def marshal_load(delegate_hash)
+      @delegate_hash = delegate_hash
+      @types = {}
+      @values = {}
+      @additional_types = {}
+      @materialized = true
+    end
+
     protected
 
-    attr_reader :types, :values, :additional_types, :delegate_hash
+    attr_reader :types, :values, :additional_types, :delegate_hash, :default
 
     def materialize
       unless @materialized
@@ -101,7 +115,7 @@ def assign_default_value(name)
       if value_present
         delegate_hash[name] = Attribute.from_database(name, value, type)
       elsif types.key?(name)
-        delegate_hash[name] = Attribute.uninitialized(name, type)
+        delegate_hash[name] = default.call(name) || Attribute.uninitialized(name, type)
       end
     end
   end

activerecord/lib/active_record/attributes.rb

@@ -253,7 +253,7 @@ def define_default_attribute(name, value, type, from_user:)
             name,
             value,
             type,
-            _default_attributes[name],
+            _default_attributes.fetch(name.to_s) { nil },
           )
         else
           default_attribute = Attribute.from_database(name, value, type)

activerecord/lib/active_record/model_schema.rb

@@ -249,7 +249,11 @@ def table_exists?
       end
 
       def attributes_builder # :nodoc:
-        @attributes_builder ||= AttributeSet::Builder.new(attribute_types, primary_key)
+        @attributes_builder ||= AttributeSet::Builder.new(attribute_types, primary_key) do |name|
+          unless columns_hash.key?(name)
+            _default_attributes[name].dup
+          end
+        end
       end
 
       def columns_hash # :nodoc:

activerecord/test/cases/attributes_test.rb

@@ -205,5 +205,49 @@ def deserialize(*)
 
       assert_equal(:bar, child.new(foo: :bar).foo)
     end
+
+    test "attributes not backed by database columns are not dirty when unchanged" do
+      refute OverloadedType.new.non_existent_decimal_changed?
+    end
+
+    test "attributes not backed by database columns are always initialized" do
+      OverloadedType.create!
+      model = OverloadedType.first
+
+      assert_nil model.non_existent_decimal
+      model.non_existent_decimal = "123"
+      assert_equal 123, model.non_existent_decimal
+    end
+
+    test "attributes not backed by database columns return the default on models loaded from database" do
+      child = Class.new(OverloadedType) do
+        attribute :non_existent_decimal, :decimal, default: 123
+      end
+      child.create!
+      model = child.first
+
+      assert_equal 123, model.non_existent_decimal
+    end
+
+    test "attributes not backed by database columns properly interact with mutation and dirty" do
+      child = Class.new(ActiveRecord::Base) do
+        self.table_name = "topics"
+        attribute :foo, :string, default: "lol"
+      end
+      child.create!
+      model = child.first
+
+      assert_equal "lol", model.foo
+
+      model.foo << "asdf"
+      assert_equal "lolasdf", model.foo
+      assert model.foo_changed?
+
+      model.reload
+      assert_equal "lol", model.foo
+
+      model.foo = "lol"
+      refute model.changed?
+    end
   end
 end