N: N relationship

Hi, I have case and a customized entities with n:n relationship, and I insert this customized entity in the case form as subgrid. It worked fine, I can select and remove items. But what I create a record in the customized entity, and tried to map to specific case through case number, it doesn't show up, I tried 1:n relationship, then it worked fine, is there anyway I can keep n:n relationship?

Thanks