Final answer:
The $push and $addToSet operators are MongoDB group accumulators used in the aggregation pipeline. $push gathers all values into an array while $addToSet ensures all values are unique, with no duplicates allowed. Both serve to modify document content during grouping.
Step-by-step explanation:
The $push and $addToSet operators are group accumulators in MongoDB, which is a NoSQL database system. These operators are used within the aggregation pipeline to modify the content of documents as they pass through the pipeline. The $push operator collects value(s) from the documents as they group, placing all of them into an array, including duplicates. On the other hand, the $addToSet operator similarly collects value(s) but ensures that all values in the resulting array are unique, effectively acting as a set where no duplicates are allowed.
For example, when grouping documents by a certain field, $push could be used to accumulate all the entries for that field, while $addToSet would accumulate only the unique entries. This is particularly useful when you need to enforce uniqueness in the grouped results.