Introduce ActiveModel::AttributeAssignment#attribute_writer_missing

Provide instances with an opportunity to gracefully handle assigning to
an unknown attribute:

```ruby
class Rectangle
  include ActiveModel::AttributeAssignment

  attr_accessor :length, :width

  def attribute_writer_missing(name, value)
    Rails.logger.warn "Tried to assign to unknown attribute #{name}"
  end
end

rectangle = Rectangle.new
rectangle.assign_attributes(height: 10) # => Logs "Tried to assign to unknown attribute 'height'"
```

By default, classes that do not override `#attribute_writer_missing`
will raise an `ActiveModel::UnknownAttributeError`.

The `attribute_writer_missing` aims to mimic the naming of
`BasicObject#method_missing`. There is also an
[ActiveModel::AttributeMethods#attribute_missing][] method, but that
pertains to attribute _access_.

[ActiveModel::AttributeMethods#attribute_missing]: https://edgeapi.rubyonrails.org/classes/ActiveModel/AttributeMethods.html#method-i-attribute_missing

Author: seanpdoyle

activemodel/CHANGELOG.md

@@ -1,2 +1,23 @@
+*   Introduce `ActiveModel::AttributeAssignment#attribute_writer_missing`
+
+    Provide instances with an opportunity to gracefully handle assigning to an
+    unknown attribute:
+
+    ```ruby
+    class Rectangle
+      include ActiveModel::AttributeAssignment
+
+      attr_accessor :length, :width
+
+      def attribute_writer_missing(name, value)
+        Rails.logger.warn "Tried to assign to unknown attribute #{name}"
+      end
+    end
+
+    rectangle = Rectangle.new
+    rectangle.assign_attributes(height: 10) # => Logs "Tried to assign to unknown attribute 'height'"
+    ```
+
+    *Sean Doyle*
 
 Please check [7-2-stable](https://github.com/rails/rails/blob/7-2-stable/activemodel/CHANGELOG.md) for previous changes.

activemodel/lib/active_model/attribute_assignment.rb

@@ -36,6 +36,27 @@ def assign_attributes(new_attributes)
 
     alias attributes= assign_attributes
 
+    # Like `BasicObject#method_missing`, `#attribute_writer_missing` is invoked
+    # when `#assign_attributes` is passed an unknown attribute name.
+    #
+    # By default, `#attribute_writer_missing` raises an UnknownAttributeError.
+    #
+    #   class Rectangle
+    #     include ActiveModel::AttributeAssignment
+    #
+    #     attr_accessor :length, :width
+    #
+    #     def attribute_writer_missing(name, value)
+    #       Rails.logger.warn "Tried to assign to unknown attribute #{name}"
+    #     end
+    #   end
+    #
+    #   rectangle = Rectangle.new
+    #   rectangle.assign_attributes(height: 10) # => Logs "Tried to assign to unknown attribute 'height'"
+    def attribute_writer_missing(name, value)
+      raise UnknownAttributeError.new(self, name)
+    end
+
     private
       def _assign_attributes(attributes)
         attributes.each do |k, v|
@@ -50,7 +71,7 @@ def _assign_attribute(k, v)
         if respond_to?(setter)
           raise
         else
-          raise UnknownAttributeError.new(self, k.to_s)
+          attribute_writer_missing(k.to_s, v)
         end
       end
   end

activemodel/test/cases/attribute_assignment_test.rb

@@ -86,6 +86,19 @@ def dup
     assert_equal "hz", error.attribute
   end
 
+  test "assign non-existing attribute by overriding #attribute_writer_missing" do
+    model_class = Class.new(Model) do
+      attr_accessor :assigned_attributes
+
+      def attribute_writer_missing(name, value) = @assigned_attributes[name] = value
+    end
+    model = model_class.new(assigned_attributes: {})
+
+    model.assign_attributes unknown: "attribute"
+
+    assert_equal({ "unknown" => "attribute" }, model.assigned_attributes)
+  end
+
   test "assign private attribute" do
     model = Model.new
     assert_raises(ActiveModel::UnknownAttributeError) do