title: find(...).populate is not a function in mongoose
body: <p>I am trying to populate two tables in mongoose and node and I receive the error that populate is not a function.</p>
  
  <p>I have search and in the documentation it seems that it does the same as I.</p>
  
  <p>Here the model:</p>
  
  <pre><code>var mongoose = require('mongoose');
  var dodleSchema = new mongoose.Schema({
      name: String,
      productorId: {type: mongoose.Schema.ObjectId, ref: "Productor"},
      description: String,
      ambassadorId: {type: mongoose.Schema.ObjectId, ref: "Ambassador"},
      accepted: { type: Boolean, default: false },
      read: { type: Boolean, default: false },
      deliveryAddress: {
          lat: String,
          lng: String
      },
      createdAt: Date,
      products: [
          {
              primaryImage: String,
              images: [],
              name: String,
              max: Number,
              min: Number,
              step: Number,
              stock: Number,
              normalPrice: Number,
              doodlePrice: Number,
              freeShipping: Boolean,
              freeShippingFrom: Number
          }
      ],
      endsAt: Date,
      packagingType: String,
      orders: [
          {
              name: String,
              email: String,
              purchases: [
                  {
                      productId: String,
                      quantity: Number
                  }
              ]
          }
      ]
  });
  module.exports = mongoose.model('Doodle', dodleSchema);
  </code></pre>
  
  <p>And then the find that I use:</p>
  
  <pre><code>router.route('/requests/:id')
  .get(function (req, res) {
      doodleCollection
          .find({
              ambassadorId: new mongodb.ObjectID(req.params.id),
              read: false
          })
          .populate('productorId')
          .toArray(function (error, results) {
              if (error) {
                  res.json({'error': "Ha habido un error en la extracción de requests"});
                  return false;
              }
  
              var alertNew = false;
  
              for (var i = 0; i &lt; results.length; i++) {
                  if (results[i].read == false) {
                      readed = true;
                      break;
                  }
              }
  
              res.json({
                  requests: results,
                  alertNew: alertNew
              });
      });
  });
  </code></pre>
  
  <p>This is the error that I get:</p>
  
  <p><a href="http://i.stack.imgur.com/6pDJN.png" rel="nofollow">Error</a></p>