It would be nice to have a list in the docs somewhere with all the things which you might need to check or change in your application when you introduce sharding. Thinks like $lookup, unique indexes, immutable shard key fields.. Some of these are in the sharding sections, some are listed on the limitations place, others only in the documentation for the operations themselves, but there does not seem to be a place which lists everything, This can make it difficult for customers (and even for us) to check what they would need to change in their application.