why does my aggregate ($lookup) function does not work in mongoose and node.js?
<p>In my <code>mongoose</code> database I have two models: <code>Users</code> and <code>Comments</code>.</p>

<p>In my <code>node.js</code> code I have:</p>

<pre><code>var Comment = require('./../models/commentsModel.js');
var User = require('./../models/usersModel.js');  
</code></pre>

<p>Now I want to construct a query with <code>$lookup</code> to return results from <code>Comments</code> and corresponding <code>Users</code> (currently <code>Comments</code> contain a field <code>user_id</code>). This is what I have so far:</p>

<pre><code>function executeQuery(query, callback) {
    query = Comment.find(query);

    Comment.aggregate([{$lookup:
    { from: "users",
        localField: "user_id",
        foreignField:"_id",
        as: "user"
    }}]);

    query.exec(function(err, comments){
        if(err) {
            callback(err);
            return;
        }
    return callback(null, comments);
    });
}
</code></pre>

<p>but I do not see any data from <code>User</code> attached to the results from <code>Comment</code>. What am I missing?</p>