Sending ids instead of embedded records for hasMany relationships

In my app on the server I have a many-to-many relationship. To simplify let us say I have a Post model and want to add different Category models to it. Categories in this example of course can have many posts.

When I save a post I want Ember Data to send the ids of the categories the user chose. If I understand it correctly, this is not currently possible with Ember Data as it a) does not support many-to-many relationships in this way and b) only sends ids for belongsTo relationships.

I searched a bit and found this solution on SO: ember.js - Save foreign key to other Model with hasMany relation - Stack Overflow

This looked a lot like what I wanted to achieve, with one caveat: It will always send ids of hasMany relationships. In my app I only want this to happen for the Post model.

So I thought about making this configurable and quickly thought of active_model_serializers which allows you to embed either full records or just ids.

Ember Data already allows embedding of full records when a mapping with embedded: 'always' exists. I enhanced the solution from the SO answer above, adding a new parameter embedded: 'ids'. Ids only get serialized now if this mapping exists.

This works fine, but leaves me with a heavily customized serializer in my app for a use case that I consider quite common.

This leads to my questions:

  1. I know that better support for many-to-many relationships is in planning. Is there anything on the horizon for my use-case or is there something already in place that I missed?
  2. If not, should I try to port back my changes to Ember Data and file a pull-request?
  3. While researching this issue, I found a third possible parameter in the code: embedded: 'load'. This seems to behave mostly like embedded: 'always' with some subtle differences I do not fully understand. What is ‘load’ meant for and what exactly is the difference to using ‘always’?
  4. Is there a place where all possible options for key mappings are documented?