How do I serialize a has-many-through relationship with Ember's JSONAPIAdapter? - ember.js

Question

I have three models: User, Group, and Membership. A User has many Groups through Memberships. I have a form to invite a new user and assign them to zero or more groups all at once.
What I Want
The JSON I expect to send to the server for POST /users looks like
{
data: {
type: 'user',
id: null,
attributes: { name: 'Sam Sample' }
},
relationships: {
memberships: {
data: [
{
type: 'membership',
id: null,
relationships: {
group: {
data: {
type: 'group',
id: 12345
}
}
}
},
{
type: 'membership',
id: null,
relationships: {
group: {
data: {
type: 'group',
id: 67890
}
}
}
}
]
}
}
}
What I Tried
I tried adding serialize: true to the relevant serializers:
// serializer:user
import ApplicationSerializer from './application';
export default ApplicationSerializer.extend({
attrs: {
memberships: { serialize: true }
}
})
// serializer:membership
import ApplicationSerializer from './application';
export default ApplicationSerializer.extend({
attrs: {
group: { serialize: true }
}
})
That gets me some of the JSON I expect, but not all of it. Specifically, I get the membership objects, but not the groups within them
{
data: {
type: 'user',
id: null,
attributes: { name: 'Sam Sample' }
},
relationships: {
memberships: {
data: [
{
type: 'membership',
id: null
},
{
type: 'membership',
id: null
}
]
}
}
}