Question: How to implement permissions for a multi tenant application?

This is a great tool and takes away the burden of creating tons of reports for individual clients but just letting them make their own reports. However, there is also my point. How am I supposed to manage the fact that one client should only see his data? Is there some hook where I could change the query?

Another way would be to provide the queries and disallow the edit, but then there one would need a way to inject the current user as immutable variable. If there is currently no way to inject the user as fixed variable and since it is open source, where would you add (allow) such a feature to be added?