How to use MongoDB in RapidMiner community version?

Not as far as I'm aware. 

Maybe ask RM for a trial licence of RM Studio professional &, if it proves worth it to your project upgrade to the full version? 

Alternative options:

• You can also investigate calling MongoDb from groovy (Excute Script), Python or R.
• You can investigate using PostgreSQL as a database which has several features that Mongo has an can arguably run faster in many cases. http://www.enterprisedb.com/postgres-plus-edb-blog/marc-linster/postgres-outperforms-mongodb-and-ushers-new-developer-reality
  

I'd recommend the PostgreSQL route myself. 

Another possibility: Using the Foreign Data Wrapper for MongoDB in PostgreSQL:
https://github.com/citusdata/mongo_fdw

With that, you don't need to export your data to PostgreSQL, just define access rules ("Foreign Tables") for your MongoDB queries.

That's even the approach MongoDB uses with their BI Connector: https://www.mongodb.com/products/bi-connector (even if that connector is proprietary).

Thanks Balázs!  I didn't know that one. 

Great to know. 

You should check out the rest of the PostgreSQL Foreign Data Wrappers: https://wiki.postgresql.org/wiki/Foreign_data_wrappers

They can do amazing things.

Maybe one day there'll be a RapidMiner Foreign Data Wrapper for executing RapidMiner processes in the database. That would be really cool.

How complicated is this?

Well, you need to have a good knowledge of PostgreSQL administration. Probably compile the extension yourself. Then you define the structure of the MongoDB query in SQL (more or less) to create the Foreign Table. It is a kind of a view to the remote "database".

Not something that can be done by novice GUI users, probably.