1 files changed, 149 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/attribute.rb b/activerecord/lib/active_record/attribute.rb
new file mode 100644
index 0000000000..88536eaac0
--- /dev/null
+++ b/activerecord/lib/active_record/attribute.rb
@@ -0,0 +1,149 @@
+module ActiveRecord
+  class Attribute # :nodoc:
+    class << self
+      def from_database(name, value, type)
+        FromDatabase.new(name, value, type)
+      end
+
+      def from_user(name, value, type)
+        FromUser.new(name, value, type)
+      end
+
+      def with_cast_value(name, value, type)
+        WithCastValue.new(name, value, type)
+      end
+
+      def null(name)
+        Null.new(name)
+      end
+
+      def uninitialized(name, type)
+        Uninitialized.new(name, type)
+      end
+    end
+
+    attr_reader :name, :value_before_type_cast, :type
+
+    # This method should not be called directly.
+    # Use #from_database or #from_user
+    def initialize(name, value_before_type_cast, type)
+      @name = name
+      @value_before_type_cast = value_before_type_cast
+      @type = type
+    end
+
+    def value
+      # `defined?` is cheaper than `||=` when we get back falsy values
+      @value = original_value unless defined?(@value)
+      @value
+    end
+
+    def original_value
+      type_cast(value_before_type_cast)
+    end
+
+    def value_for_database
+      type.type_cast_for_database(value)
+    end
+
+    def changed_from?(old_value)
+      type.changed?(old_value, value, value_before_type_cast)
+    end
+
+    def changed_in_place_from?(old_value)
+      type.changed_in_place?(old_value, value)
+    end
+
+    def with_value_from_user(value)
+      self.class.from_user(name, value, type)
+    end
+
+    def with_value_from_database(value)
+      self.class.from_database(name, value, type)
+    end
+
+    def with_cast_value(value)
+      self.class.with_cast_value(name, value, type)
+    end
+
+    def type_cast(*)
+      raise NotImplementedError
+    end
+
+    def initialized?
+      true
+    end
+
+    def ==(other)
+      self.class == other.class &&
+        name == other.name &&
+        value_before_type_cast == other.value_before_type_cast &&
+        type == other.type
+    end
+
+    protected
+
+    def initialize_dup(other)
+      if defined?(@value) && @value.duplicable?
+        @value = @value.dup
+      end
+    end
+
+    class FromDatabase < Attribute # :nodoc:
+      def type_cast(value)
+        type.type_cast_from_database(value)
+      end
+    end
+
+    class FromUser < Attribute # :nodoc:
+      def type_cast(value)
+        type.type_cast_from_user(value)
+      end
+    end
+
+    class WithCastValue < Attribute # :nodoc:
+      def type_cast(value)
+        value
+      end
+
+      def changed_in_place_from?(old_value)
+        false
+      end
+    end
+
+    class Null < Attribute # :nodoc:
+      def initialize(name)
+        super(name, nil, Type::Value.new)
+      end
+
+      def value
+        nil
+      end
+
+      def with_value_from_database(value)
+        raise ActiveModel::MissingAttributeError, "can't write unknown attribute `#{name}`"
+      end
+      alias_method :with_value_from_user, :with_value_from_database
+    end
+
+    class Uninitialized < Attribute # :nodoc:
+      def initialize(name, type)
+        super(name, nil, type)
+      end
+
+      def value
+        if block_given?
+          yield name
+        end
+      end
+
+      def value_for_database
+      end
+
+      def initialized?
+        false
+      end
+    end
+    private_constant :FromDatabase, :FromUser, :Null, :Uninitialized
+  end
+end