test.js

// ...
models: {
  a: Model.extend({
    bs: hasMany()
  }),
  b: Model.extend({
    cs: hasMany()
  }),
  c: Model                
},
seeds(server) {
  const cs1 = server.createList('c', 100);
  const bs1 = server.createList('b', 10);
  bs1.forEach((b, i) => b.cs = cs1.slice(10*i, 10*i + 10));
  console.log(server.schema.bs.all().models.map(b => b.cs.length));
  // ==> [0, 0, 0, 0, .....]
  
  server.create('a', { bs: bs1.slice(1) });
  console.log(server.schema.bs.all().models.map(b => b.cs.length));
  // ==> [0, 10, 10, 10, ....
  
  server.create('a', { bs: bs1 });
  console.log(server.schema.bs.all().models.map(b => b.cs.length));
  // ==> [10, 10, 10, 10, ....
  
  const cs2 = server.createList('c', 100);
  const bs2 = server.createList('b', 10);
  bs2.forEach((b, i) => b.update({ cs: cs2.slice(10*i, 10*i + 10)}));
  console.log(server.schema.bs.all().models.map(b => b.cs.length));
  // ==> [10, 10, 10, 10, .....]
}

// ...