From the post:
We are pleased to announce the release of MongoDB version 2.2. This release includes over 1,000 new features, bug fixes, and performance enhancements, with a focus on improved flexibility and performance. For additional details on the release:
Of particular interest to topic map fans:
Aggregation Framework
The Aggregation Framework is available in its first production-ready release as of 2.2. The aggregation framework makes it easier to manipulate and process documents inside of MongoDB, without needing to use Map Reduce, or separate application processes for data manipulation.
See the aggregation documentation for more information.
The H Open also mentions TTL (time to live) which can remove documents from collections.
MongoDB documentation: Expire Data from Collections by Setting TTL.
Have you considered “expiring” data from merges?