Array element grouping
Grouping data from documents in MongoDB is one of the most common tasks you will use the aggregation framework for. However, sometimes, you only need to group elements in an array field within each document in isolation rather than grouping data from different documents. Let's find out how we can do this efficiently in this example.
Scenario
You want to provide a report for your online game showing the total coin rewards each gaming user has accumulated. The challenge is that the source collection captures each time the game awards a user with a type of coin in a growing array field containing many elements. However, for each gamer, you want to show the total for each coin type in an array instead. An extra complication exists in that you cannot know ahead of time what all the possible coin types can be when developing the solution. For example, the game could introduce different coin types in the future (e.g., tungsten coins).
