title: Why $pull operator doesn't work on MongoDB
body: <pre><code>{
  "_id" : ObjectId("581472c3c65f3a1b3412f147"),
  "name" : "这是一级目录-----1",
  "englishName" : "zzzzz",
  "stage" : "first",
  "children" : [ 
      ObjectId("581472f1c65f3a1b3412f149"), 
      ObjectId("581472f4c65f3a1b3412f14a")
  ],
  "type" : "group",
  "order" : 50,
  "__v" : 0
  }
  </code></pre>
  
  <p>I want to remove children[1] by <code>$pull</code>, but it doesn't work.</p>
  
  <pre><code>var _id = req.fields._id;   //581472f4c65f3a1b3412f14a
  var parent_id = req.fields.parent;  //581472c3c65f3a1b3412f147
  db.Group.update({ _id: parent_id }, { $pull:{children:_id} },function(err,result){
                      console.log(result);
                      cb(err,result);
                  })
  </code></pre>
  
  <p>I have try to use <code>{$pull:{children:ObjectId(_id)}}</code> instead of  <code>{ $pull:{children:_id} }</code> ,but still failed.</p>
  
  <p><strong>Mongoose Schema</strong></p>
  
  <pre><code>var groupSchema=new Schema({
    name:String,
    englishName:String,
    order:{
      type:Number,
      default:50
    },
    type:{
      type:String,
      default:"group"
    },
    stage:String,
    children:{
      type:Array,
      default:[]
    }
  });
  </code></pre>
  
  <p>My English is very poor. Please forgive me.</p>