Hi,
thanks a lot for the offer.

if you are not familiar with git, read https://git-scm.com/book/en/v2

have a look at this page: http://hibernate.org/ogm/contribute/

Then you can start forking and cloning the project on github:
https://github.com/hibernate/hibernate-ogm

Make sure you can build it with the following command:
mvn install -pl mongodb -am -DskipDocs

Read the documentation chapter about native queries to have an idea of the limitations:
https://docs.jboss.org/hibernate/ogm/5.1/reference/en-US/html_single/#ogm-mongodb-queries-native

When everything is successful, the first step is to have a look at the class NativeQueryParser.
That is the class responsible to parse the native queries in MongoDB.

These are the steps needed to solve this issue (more or less):
1) Add a test case in NativeQueryParserTest
2) Update NativeQueryParser to make the test pass
3) Add a test in MongoDBSessionCLIQueryTest
4) Update MongoDBDialect#executeBackendQuery so that the parsed query is executed calling the appropriate method on the mongo driver
5) Once you are done send a pull request e we will review it

Let me know if you have some additional questions and make sure to use our templates for your IDE: https://github.com/hibernate/hibernate-ide-codestyles

Thanks a lot!