From MongoDB to PostgreSQL - Django
Whether the migration is easy or hard depends on a very large number of things  including how many different versions of data structures you have to accommodate.  In general you will find it a lot easier if you approach this in stages:

Ensure that all the Mongo data is consistent in structure with your RDBMS model and that the data structure versions are all the same.
Move your data.  Expect that problems will be found and you will have to go back to step 1.

The primary problems you can expect are data validation problems because you are moving from a less structured data platform to a more structured one.
Depending on what you are doing regarding MapReduce you may have some work there as well.
Could any one shed some light on how to migrate my MongoDB to PostgreSQL? What tools do I need, what about handling primary keys and foreign key relationships, etc?
I had MongoDB set up with Django, but would like to convert it back to PostgreSQL.