Hasan Savran looks at partition keys:
Selecting a partition key for your Cosmos DB is one of the most important choices you need to make for your Cosmos DB project. You really need to take your time and have a plan for your project. Where is this application will be in 1 year? 5 years? How much data are you planning to store? If your application will become popular and you start to have users all over the county or world, do you think your partition key can oversee a growth like this? These are the some of the questions you need to ask yourself. Selecting a partition key is like selecting a life partner for your project. You need a good one that will grow with your project together.
Sometimes, it does not matter how much time you spend to find a good partition key. Your document simply does not have good one. In those cases, usually the best thing you can do is combining multiple properties together and generate a unique custom property called synthetic key.
Read on for a better solution to the problem than a synthetic key.