Mock many-to-many relationship using Ember CLI Mirage

Apologies in advance: I am new to both Ember and Mirage, and I am tasked with creating acceptance tests for an existing Ember application. My challenge is in using Mirage (0.1.13) and fixtures (not factories) to mock existing one-to-many or many-to-many relationships in our data. I couldn't find any complete examples that model non-trivial relationships.

Consider the following simple many-to-many relationship: A user can have multiple roles.

The respective Ember models are:

// models/user.js
import Ember from "ember";
import DS from 'ember-data';

export default DS.Model.extend({
  email: DS.attr('string'),
  //unidirectional M-M
  roles: DS.hasMany('user-role', {inverse: null})
});

// models/user-role.js
import DS from 'ember-data';

export default DS.Model.extend({
  description: DS.attr('string')
});

Note that user-role is hyphenated.

My attempt at creating the fixtures:

// mirage/fixtures/users.js
export default [
  {id: 1, email: 'user@email.com', roles: {userRole_ids: [1]}},
  {id: 2, email: 'supervisor@email.com', roles: {userRole_ids: [1, 2]}}
];

// mirage/fixtures/userRoles.js
export default [
  {id: 1, description: 'regular user'},
  {id: 2, description: 'supervisor'}
];

//mirage/config.js
export default function() {
  //...
  //single user data
  this.get('/users/:id', function(db, request) {
    let id = request.params.id;
    return db.users.find(id);
  });
  //...
}

When I place a breakpoint in finders.js on

var payload = normalizeResponseHelper(serializer, store, typeClass, adapterPayload, id, 'findRecord');

and inspect the value of adapterPayload I see, e.g.,

email: "supervisor@email.com"
id: 2
roles: Array[2]
  0: Object:
    userRole_id: 1
  1: Object:
    userRole_id: 2

instead of the expected

email: "supervisor@email.com"
id: 2
roles: Array[2]
  0: Object
    id: 1
    description: "regular user"
  1: Object
    id: 2
    description: "supervisor"

In other words, the user role ID foreign key is not resolved by Mirage into its corresponding record. I suspect it has something to do with the hyphenation in user-role not matching my naming of the userRoles.js file.

The back-end functionality (that I am attempting to mock) is implemented using JAX-RS JPA and the Spring Framework, shown below:

// User.java
//...
@Entity
@Table(name = "User")
public class User extends BaseBizObj<User, SimplePrimaryKey<String>, UserRepository> {
    //...
    @ManyToMany(fetch = FetchType.EAGER)
    @JoinColumn(name = "Roles")
    @NotNull
    @JsonProperty
    private Set<UserRole> roles;
    //...
}

// UserRole.java
//...
@Entity
@Table(name = "UserRole")
public class User extends BaseBizObj<User, SimplePrimaryKey<String>, UserRoleRepository> {
    //...
    @Column(name = "Description")
    @NotNull
    @JsonProperty
    private String description;
    //...
}

Any help will be greatly appreciated.