partial update using mgo

The solution can be:
1. client json -> struct -> xml -> map -> database
2. Update partital with $set operator:

collection.Update(bson.M{"_id": id}, bson.M{"$set": bson.M{"name": "new Name"}})

Read more: http://docs.mongodb.org/manual/reference/operator/update/set/
3. Load the document that is going to be updated and store it in a tmp variable. Parse the user input to another variable. Overwrite the values you need to preserve before updating.