$elemMatch (projection) returns only one array element

Let’s say we have the following schemas:

 var commentSchema = new Schema({
   content: [String]
 });

 var postSchema = new Schema({
   comments: [{type: Schema.ObjectId, ref: 'Comment'}]
});

When I need to find a post with some comments restricted by id I do the following query:

 var ids = ['50478c35889a450000000001',  '50478c35889a450000000002'];

 Post.findById(req.param('post_id'))
    .select({ comments: { $elemMatch: {$in: ids }}})
    .exec(function (err, doc) {
         console.log(doc.comments); //  ['50478c35889a450000000001'] <- returns only one comment id
         done(err);
 });

Why I’m getting only the one comment id?

[aheckmann]

http://docs.mongodb.org/manual/reference/projection/elemMatch/

“Use the $elemMatch projection operator to limit the response of a query to a single matching element of an array.”

[donnut]

select() filters the fields returned by the query (findById). You should use where()

Posts.findById(req.param('post_id'))
  .where('comments', { $elemMatch: {$in: ids }})
  .exec(function(err, doc) {
     console.log(doc.comments);
     done(err);
   });