Concatenate array elements in MongoDB 3.4 with Aggregation Framework
<p><strong>What I have :</strong> </p>

<pre><code>{ "_id" : ObjectId("577dc9d61a0b7e0a40499f90"), "equ" : 123456, "key" : "p" }
{ "_id" : ObjectId("577c789b1a0b7e0a403f1b52"), "equ" : 123456, "key" : "r" }
{ "_id" : ObjectId("577b27481a0b7e0a4033965a"), "equ" : 123456, "key" : "r" }
{ "_id" : ObjectId("5779d6111a0b7e0a40282dc7"), "equ" : 123456, "key" : "o" }
</code></pre>

<p><strong>What I want :</strong> </p>

<pre><code>{ "_id" : ObjectId("5779d6111a0b7e0a40282dc7"), "equ" : 123456, "keys" : "prro" }
</code></pre>

<p><strong>What I tried :</strong></p>

<p><code>db.table.aggregate([{"$group":{"_id":0, "keys":{"$push":"$key"}}}])</code> 
returns an array and not a string:</p>

<pre><code>{"_id":0, "keys":["p","r","r","o"]}
</code></pre>

<p>Do you have any idea?</p>