Rails Active Model Serializer - Custom Serializer on AMS 0.10 - group_by - ruby-on-rails

Question

I have updated active_model_serializer from 0.9.3 to 0.10.0 as I want to use the json_api adapter. Once updated, I no longer can use both my model serializer along with my custom serializer. Here's my backtrace:
[active_model_serializers] Rendered GroupedProjectsSerializer with
ActiveModelSerializers::Adapter::JsonApi (6.03ms)
Completed 500 Internal Server Error in 24ms (ActiveRecord: 0.3ms)
NoMethodError (undefined method `read_attribute_for_serialization' for
#<Hash:0x007f90cbed0ba8>):...
I have also tried including ActiveModel::Serialization in the custom serializer which then I get another error as below:
NoMethodError (undefined method `id' for #<GroupedItemSerializer:0x007fa47012ad38>):...
adding attributes :id doesn't resolve the issue though.
Here's my setup:
Initializer:
ActiveModel::Serializer.config.adapter = :json_api
Model Serializer:
class ItemSerializer < ActiveModel::Serializer
attributes :id, :code, :name, :item_type, ....
end
Custom Serializer:
class GroupedItemSerializer < ActiveModel::Serializer
def serializable_object(options = {})
#object.map do |group_key, items|
[group_key, serialized_items(items)]
end.to_h
end
private
def serialized_items(items)
items.map do |item|
ItemSerializer.new(item, root: false)
end
end
end
Controller:
#items = Item.all.group_by(&:item_type)
# using each_serializer also doesn't help as it only uses the
# GroupedItemserializer and not applying the ItemSerializer!
render json: #items, serializer: GroupedItemSerializer
Not sure if I'm missing something in the documentation, but I've tried all the possible ways to achieve this. Any help would be appreciated.