Best practice: entity framework: where to do joins if using repository and UoW patterns

I have a user repository and partner repository. My repositories do not return IQuerables. A user entity has a partnerID. I want to use the partnerID to join the two tables Message and Table by using the repositories using Linq. However I'm not sure where to do these joins.

I know that the joins should not go in the repository. Should the join happen in the UoW? Or a service? What is best practice in terms of where I do these joins?