Can i Synchronize Apache Solr with MongoDB
If you're looking for a mongo data import handler, there are two projects on github:
Simplistic approach: https://github.com/james75/SolrMongoImporter
With support for delta imports: https://github.com/sucode/solrMongoDBImporter
I am writing a new one to cover for some missing features, and I'll publish it soon.
This related question lists a couple of possibilities. The interesting one, in my opinion, is using 10gen-labs's mongo-connector.
From their description:
The mongo-connector system is designed to hook up mongoDB to any target system. This allows all the documents in mongoDB to be stored in some other system, and both mongo and the target system will remain in sync while the connector is running. It has been tested with python 2.7 and python 3.
Furthermore, they have a section describing integration with Solr.
It seems to have been updated two months ago, so it's not a derelict project. I think the fact that it is a "10gen-labs" project is also a point in its favor, since 10gen is the old name of the company that makes MongoDB.