Hacky solution for https://github.com/SeyZ/jsonapi-serializer/issues/58

example.js

import { Serializer, Deserializer } from 'jsonapi-serializer';

return new Deserializer(relationshipProxy).deserialize(response.data);

relationshipProxy.js

/**
 * This proxy ensures that every relationship is serialized to an object of the form {id: x}, even if that relationship
 * doesn't have included data
 */
const specialOpts = [ 'transform', 'keyForAttribute', 'id', 'typeAsAttribute' ];
const relationshipProxy = new Proxy({}, {
  has(target, key) {
    // Pretend to have all keys except certain ones with special meanings
    return !specialOpts.includes(key);
  },
  get(target, key, receiver) {
    if (specialOpts.includes(key)) {
      return undefined;
    }

    return {
      valueForRelationship(data, included) {
        // If we have actual included data use it, but otherwise just return the id in an object
        if (included){
          return included;
        } else {
          return { id: data.id };
        }
      },
    };
  },
});