Javascript Backbone.js过滤器集合返回一个对象，而不是集合

我试图过滤模型集合，但过滤器似乎只返回一个对象，而不是一个集合。从我所读到的内容来看，将filter函数包装在uz（）中将强制它使用下划线.js内置过滤器，并返回相同的类型。但这似乎不起作用。代码在下面，有什么想法吗

    var clients = this.get('clients')

    if (clients instanceof Backbone.Collection) {
      console.log('clients is a collection');
    } else if (_.isObject(clients)) {
      console.log('clients is an object');
    } else if (_.isArray(clients)) {
      console.log('clients is an array');
    } else {
      console.log('clients is not known');
    }

    clients = _(clients.filter(function (client) {
      return client.get('case_studies').length;
    }));

    if (clients instanceof Backbone.Collection) {
      console.log('clients is a collection');
    } else if (_.isObject(clients)) {
      console.log('clients is an object');
    } else if (_.isArray(clients)) {
      console.log('clients is an array');
    } else {
      console.log('clients is not known');
    }

这是我的输出：

    clients is a collection
    clients is an object 

---

假设您像这样实例化了

客户机
集合：

var Client = Backbone.Model.extend({});
var Clients = Backbone.Collection.extend({
  model: Client
});
var clients = new Clients();

那么你需要做的就是：

clients = new Clients(clients.filter(function (client) {
  return client.get('case_studies').length
}));

假设您像这样实例化了
客户机
集合：

var Client = Backbone.Model.extend({});
var Clients = Backbone.Collection.extend({
  model: Client
});
var clients = new Clients();

那么你需要做的就是：

clients = new Clients(clients.filter(function (client) {
  return client.get('case_studies').length
}));

完美地工作，只是在结尾缺少一个括号。完美地工作，只是在结尾缺少一个括号。