title: Mongoose query populate match id of find elements
body: <p>I'm trying to populate a model with data from another model. Those two models looks like this:</p>
  
  <pre><code>var postSchema = mongoose.Schema({
      _comments: { type: mongoose.Schema.Types.ObjectId, ref: 'Comment' },
      type: String,
      body: String,
  });
  
  var commentSchema = mongoose.Schema({
      id_post: mongoose.Schema.Types.ObjectId,
      body: String,
  });
  </code></pre>
  
  <p>I want to find all <code>posts</code> and populate them with <code>comments</code> that have <code>id_post</code> == <code>_id</code> from founded Posts. Something like this: </p>
  
  <pre><code>Post.find({}).populate({
      path: '_comments',
      select: 'body',
      match: { post_id: Post._id }
      options: { limit: 5 }
  })
  .exec(function (err, posts){...});
  </code></pre>