title: Mongoose Populate not returning related data
body: <p>I have the following Models:</p>
  
  <pre><code>const mongoose = require('mongoose');
  mongoose.Promise = global.Promise;
  
  const imputerSchema = new mongoose.Schema({
    dninie: {
      type: String,
      trim: true,
    },
    name: {
      type: String,
      trim: true,
      required: true,
    },
    lastname: {
      type: String,
      trim: true,
      required: true,
    },
    second_lastname: {
      type: String,
      trim: true,
    },
    phone : {
      type: Number,
      unique: true,
      trim: true,
    },
    qr: {
      type : String,
      unique: true,
      default: Date.now
    }
  });
  
  module.exports = mongoose.model('Imputer', imputerSchema)
  
  const mongoose = require('mongoose');
  mongoose.Promise = global.Promise;
  
  const imputationSchema = new mongoose.Schema({
    qr: {
      type: String,
      trim: true,
      required: true,
      ref: 'Imputer',
    },
    imputation_date: {
      type: String,
      default: Date.now,
      required: true,
    },
    coordinates: {
      type: [Number, Number],
      index: '2d',
    },
  });
  
  module.exports = mongoose.model('Imputation', imputationSchema);
  </code></pre>
  
  <p>and I trying to make a query like this:</p>
  
  <pre><code>Imputation.find()
          .populate('imputer.qr')
          .exec()
          .then(docs =&gt; console.log(docs));
  </code></pre>
  
  <p>I also try</p>
  
  <pre><code>   Imputation.find()
              .populate('imputer')
              .exec()
              .then(docs =&gt; console.log(docs));
  </code></pre>
  
  <p>But I'm only got the documents on the imputation model without the field on the imputers model.</p>
  
  <p>Here are some screenshots of how the documents look<a href="https://i.stack.imgur.com/nRqfx.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/nRqfx.png" alt="enter image description here"></a>
  <a href="https://i.stack.imgur.com/BYUDP.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/BYUDP.png" alt="enter image description here"></a></p>