Mongodb now supports effectivly "Joins" in its aggregation framework, so it can do some relational style data representation. The latest versions also support transactions.
Also in their blogpost [1], about $lookup they say:
We’re still concerned that $lookup can be misused to treat MongoDB like a relational database. But instead of limiting its availability, we’re going to help developers know when its use is appropriate, and when it’s an anti-pattern. In the coming months, we will go beyond the existing documentation to provide clear, strong guidance in this area.