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

1 files changed, 21 insertions, 7 deletions

diff --git a/activerecord/lib/active_record/attribute_set/builder.rb b/activerecord/lib/active_record/attribute_set/builder.rb
index 24a255efc1..4ffd39d82d 100644
--- a/activerecord/lib/active_record/attribute_set/builder.rb
+++ b/activerecord/lib/active_record/attribute_set/builder.rb
@@ -3,11 +3,12 @@ require 'active_record/attribute'
 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 @@ module ActiveRecord
           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 @@ module ActiveRecord
       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 @@ module ActiveRecord
       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