pushPayload can not update property of model which is set previously

As the following, user has serviceinfo, which is of userService type:

User = DS.Model.extend({
    serviceinfo: belongsTo('userService', {async: true})
});

UserService = DS.Model.extend({
   online: attr('number', {defaultValue: 0})
});

When the app start, online of user with id '1' is set to 1:

self.get('store').find('user', '1').then(function(user){
     user.get('serviceinfo').then(function(serviceinfo){
          serviceinfo.set('online', 1);
    });
}); 

and after a while, pushPayload is called when a message with userService data(id is '1', online is 2 ) is received:

self.get('store').pushPayload('userService', {userService: msg});

but the state gotten after that by:

self.get('store').getById('user', '1').get('serviceinfo.online');

is still '1', that is to say, pushPayload can not change online property in userService model if 'set' is called firstly.

What is the problem?