Yii2 Nested relations in Rest results - yii2

Question

I have a customer table and a user table, they have a M:M relationship set in user_has_customer.
The entire application is a Rest API, and i use extraFields() and GET customers?expand=users to get the connected users when looking at customers.
user_has_customer stores some additional information on when the user was assigned to the customer and a few other fields, and several fields can have the same combination of user and customer. To be able to update or delete a relation, I need the relation id (user_has_customer.id) to be visible when looking at customers.
This function will return the customer, and its related users:
public function getUsers() {
return $this->hasMany(User::className(), ['id' => 'user_id'])
->viaTable('user_has_customer', ['customer_id' => 'id'])
->select(['id', 'username']);
}
This function will return the user-customer relationship:
public function getUserRelations() {
return $this->hasMany(UserHasCustomer::className(), ['customer_id' => 'id'])
->select(["id",
"customer_id",
"user_id",
"created_by",
"created_at"]);
}
But i would like to know how i can nest these relations, so the result looks like this:
{
"id": 11148,
....
"users": {
"id": 1,
"user_id": 1,
"added": "2017-08-01 22:23:24"
"user": {
"id": 1,
"username": "admin"
}
}
}
or even like this (like a MySQL join):
{
"id": 11148,
....
"users": {
"id": 1,
"user_id": 1,
"added": "2017-08-01 22:23:24"
"username": "admin"
}
}
For some reason this will not work (the created SQL is correct, but the AR refuse to show the fields from user in the result):
public function getUserRelations() {
return $this->hasMany(UserHasCustomer::className(), ['customer_id' => 'id'])
->joinWith('user', true)
- ->select('*');
}