Document Model delcared attributes

Author: bf4

docs/howto/serialize_poro.md

@@ -42,4 +42,32 @@ end
 
 The default serializer would be `MyModelSerializer`.
 
+*IMPORTANT*: There is a surprising behavior (bug) in the current implementation of ActiveModelSerializers::Model that
+prevents an accessor from modifying attributes on the instance.  The fix for this bug
+is a breaking change, so we have made an opt-in configuration.
+
+New applications should set:
+
+```ruby
+ActiveModelSerializers::Model.derive_attributes_from_names_and_fix_accessors
+```
+
+Existing applications can use the fix *and* avoid breaking changes
+by making a superclass for new models. For example:
+
+```ruby
+class SerializablePoro < ActiveModelSerializers::Model
+  derive_attributes_from_names_and_fix_accessors
+end
+```
+
+So that `MyModel` above would inherit from `SerializablePoro`.
+
+`derive_attributes_from_names_and_fix_accessors` prepends the `DeriveAttributesFromNamesAndFixAccessors`
+module and does the following:
+
+- `id` will *always* be in the attributes. (This is until we separate out the caching requirement for POROs.)
+- Overwrites the `attributes` method to that it only returns declared attributes.
+ `attributes` will now be a frozen hash with indifferent access.
+
 For more information, see [README: What does a 'serializable resource' look like?](../../README.md#what-does-a-serializable-resource-look-like).

lib/active_model_serializers/model.rb

@@ -56,14 +56,6 @@ def self.included(base)
         base.attributes :id
       end
 
-      # Override the initialize method so that attributes aren't processed.
-      #
-      # @param attributes [Hash]
-      def initialize(attributes = {})
-        @errors = ActiveModel::Errors.new(self)
-        super
-      end
-
       # Override the +attributes+ method so that the hash is derived from +attribute_names+.
       #
       # The the fields in +attribute_names+ determines the returned hash.