Use the database type to deserialize enum

This fixes incorrect assumptions made by e991c7b that we can assume the
DB is already casting the value for us. The enum type needs additional
information to perform casting, and needs a subtype.

I've opted not to call `super` in `cast`, as we have a known set of
types which we accept there, and the subtype likely doesn't accept them
(symbol -> integer doesn't make sense)

Close #23190

activerecord/lib/active_record/enum.rb

@@ -105,9 +105,10 @@ def inherited(base) # :nodoc:
     end
 
     class EnumType < Type::Value # :nodoc:
-      def initialize(name, mapping)
+      def initialize(name, mapping, subtype)
         @name = name
         @mapping = mapping
+        @subtype = subtype
       end
 
       def cast(value)
@@ -124,7 +125,7 @@ def cast(value)
 
       def deserialize(value)
         return if value.nil?
-        mapping.key(value)
+        mapping.key(subtype.deserialize(value))
       end
 
       def serialize(value)
@@ -139,7 +140,7 @@ def assert_valid_value(value)
 
       protected
 
-      attr_reader :name, :mapping
+      attr_reader :name, :mapping, :subtype
     end
 
     def enum(definitions)
@@ -158,7 +159,9 @@ def enum(definitions)
         detect_enum_conflict!(name, name)
         detect_enum_conflict!(name, "#{name}=")
 
-        attribute name, EnumType.new(name, enum_values)
+        decorate_attribute_type(name, :enum) do |subtype|
+          EnumType.new(name, enum_values, subtype)
+        end
 
         _enum_methods_module.module_eval do
           pairs = values.respond_to?(:each_pair) ? values.each_pair : values.each_with_index

activerecord/test/cases/enum_test.rb

@@ -411,4 +411,14 @@ def self.name; 'Book'; end
     assert book.proposed?, "expected fixture to default to proposed status"
     assert book.in_english?, "expected fixture to default to english language"
   end
+
+  test "uses default value from database on initialization" do
+    book = Book.new
+    assert book.proposed?
+  end
+
+  test "uses default value from database on initialization when using custom mapping" do
+    book = Book.new
+    assert book.hard?
+  end
 end

activerecord/test/models/book.rb

@@ -14,6 +14,7 @@ class Book < ActiveRecord::Base
   enum author_visibility: [:visible, :invisible], _prefix: true
   enum illustrator_visibility: [:visible, :invisible], _prefix: true
   enum font_size: [:small, :medium, :large], _prefix: :with, _suffix: true
+  enum cover: { hard: 'hard', soft: 'soft' }
 
   def published!
     super

activerecord/test/schema/schema.rb

@@ -104,6 +104,7 @@ def except(adapter_names_to_exclude)
     t.column :author_visibility, :integer, default: 0
     t.column :illustrator_visibility, :integer, default: 0
     t.column :font_size, :integer, default: 0
+    t.column :cover, :string, default: 'hard'
   end
 
   create_table :booleans, force: true do |t|