Database
[{"_id":{"$oid":"605cee2f08588cb6263c33a4"},"userID":"9346","firstName":"James","sessionsLog":{"date":[],"timestamp":[],"duration":[]}},{"_id":{"$oid":"605cee825ef711180adbc141"},"userID":"4778","firstName":"Rob","sessionsLog":{"date":[],"timestamp":[],"duration":[]}}]
Query
db.collection.update({"userID":"9346"},{$push:{"sessionsLog.date":{a:1,b:2}}})