why node-orm2's hasMany return the wrong result?

I have tow models: Order and Service:

Order = db.define('order', {
  id,
  ....
})

Service = db.define('service', {
  id,
  ....
})

and I have make a hasMany to expression order could have many services:

Order.hasMany('service', Service, {
  service_id: 'number',
  order_id: 'number'
}, {
  autoFetch: true
});

when I use:

order.addService(server)

everything gos well, but when I use getService:

order.getService(function(err, services){
    //here, the services is a one element array,  this element is the first line of order table. I can't understand it.
})

Could anyone tell me how to debug or fix this?

Thanks!