Upsert of Mongo API not working with Azure Cosmos DB
Yes, based on the doc MongoDB query language support , upsert() command is not supported by cosmos db mongo api. As I known, no shortcuts here so far. You need to encapsulate methods to determine whether a document exists, and then decide to insert or update.
Or, just to declare that the Cosmos Document DB SDK supports Upsert method. Please refer to the case: How can I perform an UPSERT using Azure DocumentDB?.
Cosmos Document DB is a good choice if you could do data migrations.
Hope it helps you.