mongo join between two collections
<p>I have two mongo collections: </p>

<ol>
<li><p>defn</p>

<pre><code>"_id" : ObjectId("8570bebcb7db3"), 
"fields" : [    {      
"control" : {   "appearance" : "field-list" },  
"children" : [ {       "bind" : {      "required" : "yes" },   ...
</code></pre></li>
<li><p>data</p>

<pre><code>"_id" : ObjectId("1570bf18a7db"),
"defn" : ObjectId("8570bebcb7db3"),
"data" : {
        "country" : "",
        "age" : 1,
        "age_unit" : "years", 
},
"label" : "type"
</code></pre></li>
</ol>

<p>that are joined where defn._id =data.defn</p>

<p>How can I write a query to return data.label for defn._id?
The query would be of the form
db.defn.find({ data.label where defn._id= "X"})</p>