Issues with $push in mongo

I am running into a problem and I am not sure how to fix it. Any help you can provide would be great, I am sure it is something stupid I am doing wrong. :)

I am trying to $push some items into an array titled "subscribed_tribes". Here is my document prior to any pushes:

{ 
"_id" : ObjectId("5693fc1f6890f14daa0e26e1"), 
"password" : "7CAE......", 
"salt" : "748D...", 
"created" : ISODate("2016-01-11T19:01:51.000+0000"), 
"display_name" : "2686....", 
"my_tribes" : "", 
"subscribed_tribes" : ""
}

---

When I run the following c code nothing happens (no changes).

        collection = mongoc_client_get_collection (client, "STribe", "users");

        query = bson_new ();
        bson_oid_t oid;
        char *techSupportID = "5693c0196890f159c1741bb1";
        bson_oid_init_from_string (&oid, techSupportID);
        query = BCON_NEW ("_id", BCON_OID(&oid));

        // Find the document
        cursor = mongoc_collection_find (collection, MONGOC_QUERY_NONE, 0, 0, 0, query, NULL, NULL);

        update = bson_new ();
        //BSON_APPEND_UTF8 (update, "testing", "blah");
        //mongoc_collection_update (collection, MONGOC_UPDATE_NONE, query, update, NULL, &error);


        // Assemble query
        update = BCON_NEW ("$push", 
            "{", 
                "subscribed_tribes", 
                    "{", 
                        "tribe_id",     BCON_UTF8 (tribe_id), 
                        "tribe_key",  BCON_UTF8 (passphrase), 
                    "}", 
            "}");
        mongoc_collection_update (collection, MONGOC_UPDATE_NONE, query, update, NULL, &error);

If I uncomment the two lines of code (bson_append and mongc_collection_update) then the entire document is overwritten and replaced with:

{ 
"_id" : ObjectId("5693c0196890f159c1741bb1"), 
"testing" : "blah", 
"subscribed_tribes" : [
    {
        "tribe_id" : "5624200d4bacd3940b8b2d62", 
        "tribe_key" : "27D719EDC7A59...."
    }
]
}

Any ideas why the original code doesn't add the array item into subscribed_tribes correctly? Secondly why does the inclusion of the uncommented lines overwrite the entire document?

Once again I am sure I am doing something stupid, but I am just not sure what.

subscribed_tribes is currently a string, not an array. That's why you can't use $push on it.

If you inspect the populated error result, you should be getting an error message about that.

So your doc needs to look like this for $push to work:

{ 
"_id" : ObjectId("5693fc1f6890f14daa0e26e1"), 
"password" : "7CAE......", 
"salt" : "748D...", 
"created" : ISODate("2016-01-11T19:01:51.000+0000"), 
"display_name" : "2686....", 
"my_tribes" : "", 
"subscribed_tribes" : []
}