To perform "upsert" operation on a sharded MongoDB cluster, the query MUST exactly match the shard key.

For example:

update( {a:1, b:2}, { $setOnInsert { c : 3, d : 4}}, { upsert: true})

The above update does not work if the shard is key is for example

{a: 1, _id: 1}. 

Is this the designed behavior? If so, why cannot I not find the detailed example or warning for this? Since the shard kay cannot be modified, this is causing huge problems. It just completely broke our migration from a stand-alone server to the sharded cluster.