failing test for polymorphic relationship being set up correctly

I am bit confused by this use case. You are setting up the Post to always have a contact, but the only contacts there are are emails? Seems like you would either define the Post to only have emails or if there are other cases possible define contact as having posts. This case where you target a parent type from one side but the child type from the other seems a bit contrived? I am happy to help/find a solution, but would first like to understand the issue a bit more