Can't find documents searching by ObjectId using Mongoose

A couple tips:

• Try running the same query from mongodb at the command line, see if you get any results.
• Is the "campaign_id" defined as an ObjectId in your schema? If so, try searching using the ObjectId type.

For example:

var ObjectId = require('mongoose').Types.ObjectId; 
var query = { campaign_id: new ObjectId(campaign._id) };

Just to improve the previous (correct) answer, i use on my projects :

String.prototype.toObjectId = function() {
  var ObjectId = (require('mongoose').Types.ObjectId);
  return new ObjectId(this.toString());
};

// Every String can be casted in ObjectId now
console.log('545f489dea12346454ae793b'.toObjectId());

Instead of using ObjectId to find by comparing your parameters simply use

Campaign.findById {req.param('client_id'),function(err,docs)}....

when finding docs using objectId findById is the most efficient way of all...