Let's have a collection sharded by key

Then let's find an answer by its primary key:
answer = Answer.find('4c91524c9b3b494747000019')

Try to update
answer.is_open = 1
answer.save

Save sends this
Answer.collection.update({:_id => answer.id},
{"$set" => {:opn => 1,
:updated_at => Time.now}})

This operation silently fails on Mongo side and Mongoid doesn't capture this.

Should either fail as well, or send shard key along
Answer.collection.update({:_id => answer.id, :answerer => @viewer.id},
{"$set" => {:is_open => 1,
:updated_at => Time.now}})

We could be able to specify shard key in the model.