Amazon DocumentDB (with MongoDB compatibility) adds support for $mergeObjects and $reduce

Amazon DocumentDB (with MongoDB compatibility) is a scalable, highly durable, and fully managed database service for operating mission-critical MongoDB workloads.

Amazon DocumentDB continues to increase compatibility with MongoDB and today added support for the following MongoDB APIs in DocumentDB 3.6 and 4.0:

• $mergeObjects: Aggregation pipeline operator that combines multiple documents into a single document
• $reduce: Aggregation operator that applies an expression to each element in an array and combines them into a single value

These capabilities are supported in all regions where Amazon DocumentDB is available including the Amazon Web Services China (Ningxia) Region, operated by NWCD. All supported MongoDB APIs and aggregation pipeline capabilities for Amazon DocumentDB can be found here.