In a fixed schema database, all of the data in the database must conform to the current version of the schema. However, in MongoDB, a collection may include documents that are structured differently, ...

MongoDB 3.2 adds a number of new tools and features to help data analysts, DBAs and operations teams integrate the database within their existing processes and toolsets.

Schema design is the critical path to using MongoDB efficiently and effectively. Just as with your RDBMS, you may have to make compromises in order to achieve better atomicity or scale.