Add read_attribute_for_database

The `BeforeTypeCast` module defines `read_attribute_before_type_cast`,
`attributes_before_type_cast`, and `*_before_type_cast` attribute
methods.  It also defines `attributes_for_database` and `*_for_database`
attribute methods, but no corresponding `read_attribute_for_database`
method.

This commit adds the missing `read_attribute_for_database` method.

activerecord/lib/active_record/attribute_methods/before_type_cast.rb

@@ -52,6 +52,23 @@ def read_attribute_before_type_cast(attr_name)
         attribute_before_type_cast(name)
       end
 
+      # Returns the value of the attribute identified by +attr_name+ after
+      # serialization.
+      #
+      #   class Book < ActiveRecord::Base
+      #     enum status: { draft: 1, published: 2 }
+      #   end
+      #
+      #   book = Book.new(status: "published")
+      #   book.read_attribute(:status)              # => "published"
+      #   book.read_attribute_for_database(:status) # => 2
+      def read_attribute_for_database(attr_name)
+        name = attr_name.to_s
+        name = self.class.attribute_aliases[name] || name
+
+        attribute_for_database(name)
+      end
+
       # Returns a hash of attributes before typecasting and deserialization.
       #
       #   class Task < ActiveRecord::Base

activerecord/test/cases/attribute_methods_test.rb

@@ -215,7 +215,17 @@ def setup
     end
   end
 
-  test "read attributes_for_database" do
+  test "read_attribute_for_database" do
+    topic = Topic.new(content: ["ok"])
+    assert_equal "---\n- ok\n", topic.read_attribute_for_database("content")
+  end
+
+  test "read_attribute_for_database with aliased attribute" do
+    topic = Topic.new(title: "Hello")
+    assert_equal "Hello", topic.read_attribute_for_database(:heading)
+  end
+
+  test "attributes_for_database" do
     topic = Topic.new
     topic.content = { "one" => 1, "two" => 2 }
 
@@ -226,7 +236,7 @@ def setup
     assert_not_equal topic.attributes, before_type_cast_attributes
   end
 
-  test "read attributes_after_type_cast on a date" do
+  test "read attributes after type cast on a date" do
     tz = "Pacific Time (US & Canada)"
 
     in_time_zone tz do