1 files changed, 241 insertions, 0 deletions
diff --git a/activemodel/lib/active_model/attribute.rb b/activemodel/lib/active_model/attribute.rb
new file mode 100644
index 0000000000..43130c37c5
--- /dev/null
+++ b/activemodel/lib/active_model/attribute.rb
@@ -0,0 +1,241 @@
+# frozen_string_literal: true
+
+module ActiveModel
+  class Attribute # :nodoc:
+    class << self
+      def from_database(name, value, type)
+        FromDatabase.new(name, value, type)
+      end
+
+      def from_user(name, value, type, original_attribute = nil)
+        FromUser.new(name, value, type, original_attribute)
+      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, original_attribute = nil)
+      @name = name
+      @value_before_type_cast = value_before_type_cast
+      @type = type
+      @original_attribute = original_attribute
+    end
+
+    def value
+      # `defined?` is cheaper than `||=` when we get back falsy values
+      @value = type_cast(value_before_type_cast) unless defined?(@value)
+      @value
+    end
+
+    def original_value
+      if assigned?
+        original_attribute.original_value
+      else
+        type_cast(value_before_type_cast)
+      end
+    end
+
+    def value_for_database
+      type.serialize(value)
+    end
+
+    def changed?
+      changed_from_assignment? || changed_in_place?
+    end
+
+    def changed_in_place?
+      has_been_read? && type.changed_in_place?(original_value_for_database, value)
+    end
+
+    def forgetting_assignment
+      with_value_from_database(value_for_database)
+    end
+
+    def with_value_from_user(value)
+      type.assert_valid_value(value)
+      self.class.from_user(name, value, type, original_attribute || self)
+    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 with_type(type)
+      if changed_in_place?
+        with_value_from_user(value).with_type(type)
+      else
+        self.class.new(name, value_before_type_cast, type, original_attribute)
+      end
+    end
+
+    def type_cast(*)
+      raise NotImplementedError
+    end
+
+    def initialized?
+      true
+    end
+
+    def came_from_user?
+      false
+    end
+
+    def has_been_read?
+      defined?(@value)
+    end
+
+    def ==(other)
+      self.class == other.class &&
+        name == other.name &&
+        value_before_type_cast == other.value_before_type_cast &&
+        type == other.type
+    end
+    alias eql? ==
+
+    def hash
+      [self.class, name, value_before_type_cast, type].hash
+    end
+
+    def init_with(coder)
+      @name = coder["name"]
+      @value_before_type_cast = coder["value_before_type_cast"]
+      @type = coder["type"]
+      @original_attribute = coder["original_attribute"]
+      @value = coder["value"] if coder.map.key?("value")
+    end
+
+    def encode_with(coder)
+      coder["name"] = name
+      coder["value_before_type_cast"] = value_before_type_cast unless value_before_type_cast.nil?
+      coder["type"] = type if type
+      coder["original_attribute"] = original_attribute if original_attribute
+      coder["value"] = value if defined?(@value)
+    end
+
+    protected
+
+      attr_reader :original_attribute
+      alias_method :assigned?, :original_attribute
+
+      def original_value_for_database
+        if assigned?
+          original_attribute.original_value_for_database
+        else
+          _original_value_for_database
+        end
+      end
+
+    private
+      def initialize_dup(other)
+        if defined?(@value) && @value.duplicable?
+          @value = @value.dup
+        end
+      end
+
+      def changed_from_assignment?
+        assigned? && type.changed?(original_value, value, value_before_type_cast)
+      end
+
+      def _original_value_for_database
+        type.serialize(original_value)
+      end
+
+      class FromDatabase < Attribute # :nodoc:
+        def type_cast(value)
+          type.deserialize(value)
+        end
+
+        def _original_value_for_database
+          value_before_type_cast
+        end
+      end
+
+      class FromUser < Attribute # :nodoc:
+        def type_cast(value)
+          type.cast(value)
+        end
+
+        def came_from_user?
+          !type.value_constructed_by_mass_assignment?(value_before_type_cast)
+        end
+      end
+
+      class WithCastValue < Attribute # :nodoc:
+        def type_cast(value)
+          value
+        end
+
+        def changed_in_place?
+          false
+        end
+      end
+
+      class Null < Attribute # :nodoc:
+        def initialize(name)
+          super(name, nil, Type.default_value)
+        end
+
+        def type_cast(*)
+          nil
+        end
+
+        def with_type(type)
+          self.class.with_cast_value(name, nil, type)
+        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:
+        UNINITIALIZED_ORIGINAL_VALUE = Object.new
+
+        def initialize(name, type)
+          super(name, nil, type)
+        end
+
+        def value
+          if block_given?
+            yield name
+          end
+        end
+
+        def original_value
+          UNINITIALIZED_ORIGINAL_VALUE
+        end
+
+        def value_for_database
+        end
+
+        def initialized?
+          false
+        end
+
+        def with_type(type)
+          self.class.new(name, type)
+        end
+      end
+
+      private_constant :FromDatabase, :FromUser, :Null, :Uninitialized, :WithCastValue
+  end
+end