Association integration in a REST API with Rails and Angular 2/4 - ruby-on-rails

Question

I'm working in a web application with Angular 2/4 on client side and a Rails application on the server side. In the rails side I have a model Product that has the following serializer:
class ProductSerializer < ActiveModel::Serializer
attributes :id, :weight
belongs_to :brand
end
This serializer generates:
{
"id": 1,
"weight": 62.0,
"brand": {
"id": 1,
"name": "foo"
}
}
Then, in Angular I have this class to parse the JSON:
export class Produto implements Model {
id: number;
weight: number;
brand: Brand;
}
Until here is everything ok, the issue began when the user alter the Product and the Angular has to send the Product altered to update route in Rails. If I just send the TypeScript object in the body of PUT or POST, the generated JSON will have the same pattern that the one received from Rails. But my rails app was expecting the following pattern:
{
"id": 1,
"weight": 62.0,
"brand_id": 1
}
In the ProductController I have the method product_params that I did this workaround to accept the pattern sended by Angular:
def produto_params
params[:brand_id] = params[:brand][:id] if params[:brand][:id]
params.delete(:brand) if params[:brand]
params.permit(:weight, :brand_id)
end
All said, this looks ugly and not scalable at all. This is just one association, if my product scale to 5 association this will grow big. Besides that, I can have many others situations like that with others models.
I think that I'm doing structure wrong, but where is wrong and where is right is obscure to me, so I need some help to structure in a scalable way.
Some considerations: Both Angular app and Rails app are mine and they are in early development, so I can do structural changes with few troubles. I learning Ruby, Rails, TypeScript and Angular with this project, but I'm trying to reuse code the maximum I can, so what want to achieve is a structure that I don't need to write serializer for every model in both sides. A approach that thought was a generic serializer in Angular, using decoratos in the fields that carry object from association, so overriding the toJSON() method I would transform a brand in a brand_id (don't know if is even possible), this would be a bad approach?
Thanks in advance.