title: MongoDB: find by one objectId in array
body: <p>I'm having trouble with query the DB, I want to get all element that the have for example: admin "54bd13864ec56c7c12310a79" in  the admins array,</p>
  
  <p>tried it with "$in" but it didn't worked, could it be related to the fact that its an ObjectId?</p>
  
  <pre><code>trainerId = '54bd13864ec56c7c12310a79'
  GroupSchema.find({'admins': { $in: [ trainerId ] }}
  </code></pre>
  
  <p>This is my db:</p>
  
  <pre><code>{
     "_id" : ObjectId("54b93e8e3801ae381e3433be"),
     "groupName" : "Developers Groups",
     "createdBy" : "Ido",
     "creationDate" : "Jan 16 2015",
     "users" : [ 
         ObjectId("54b932c7ac3ec34a85e6246c")
     ],
     "admins" : [ 
         ObjectId("54b932c7ac3ec34a85e6246c"), 
         ObjectId("54bd13864ec56c7c12310a79")
     ],
     "__v" : 0
  }
  </code></pre>
  
  <p>The Schema model is:</p>
  
  <pre><code>module.exports = mongoose.model('Groups' ,
  {
      groupName: String,
      createdBy: String,
      creationDate: String,
      admins: [{ type : mongoose.Schema.Types.ObjectId, ref: 'Users' }],
      users: [{ type : mongoose.Schema.Types.ObjectId, ref: 'Users' }]
  }
  );
  </code></pre>