Mongodb: dynamically exclude fields from update function

Question

I have the following Meteor method:

  'updateDoc': function (doc, obj) {
      Movies.update(
        {_id: doc},
        {$set: {field1: obj.key1, field2: obj.key2, field3: obj.key3} }
      );
    },

The problem is that obj creates dynamically and, for example, may not include key2. Is there any way to exclude appropriate field from update function to avoid overwriting to null in that case?