title: Mongoose select with populate not working
body: <p>I am making a query in mogoose and if i add any parameter on a query select, the populate parameter goes missing for example i have the following schemas:</p>
  
  <p>Department:</p>
  
  <pre><code>var mongoose = require('mongoose');
  var schema = mongoose.Schema({
      name: {type:String,required: true,index: {unique: true}} ,
      text: String
  })
  
  module.exports=mongoose.model('Department',schema);
  </code></pre>
  
  <p>Employee:</p>
  
  <pre><code>var mongoose = require('mongoose');
  var ObjectId=mongoose.Schema.ObjectId;
  
  var schema = mongoose.Schema({
      name: {type:String,required: true} ,
      lastName: {type:String} ,
      birthday:Date,
      email:{type:String,required: true,index: {unique: true}},
      _department:{type:ObjectId,ref:'Department'},
      isUser:Boolean
  },{ strict:false});    
  module.exports=mongoose.model('Employee',schema);
  </code></pre>
  
  <p>if i make:</p>
  
  <pre><code>var query=mongoose.model('Employee').find();
  query.select('email').populate('_department','name');
  query.exec(function(err,data){
      console.log(data);
  });
  </code></pre>
  
  <p>I get the following ouput</p>
  
  <pre><code> [ { email: 'email@email.com.br', _id: 532e570864803bf505e51c81 } ]
  </code></pre>
  
  <p><strong>I would expect this:</strong></p>
  
  <pre><code>[ { _department: { _id: 532c77c3485925d806436981, name: 'bar' },
    email: 'email@email.com.br',
    _id: 532e570864803bf505e51c81,
    __v: 0 } ]
  </code></pre>
  
  <p>If i make the following:</p>
  
  <pre><code>var query=mongoose.model('Employee').find();
  query.populate('_department','name');
  query.exec(function(err,data){
      console.log(data);
  });
  </code></pre>
  
  <p>I get this output  </p>
  
  <pre><code>[ { _department: { _id: 532c77c3485925d806436981, name: 'bar' },
     name: 'mimimi',
    email: 'email@email.com.br',
    _id: 532e570864803bf505e51c81,
    __v: 0 } ]
  </code></pre>
  
  <p>What makes me wonder that the select is breaking the populate. 
  My mongoose version is 3.8.8</p>