Rails - Find By with 2 fields?

I have the following in a controller

def update
    @permission = Permission.find_by_user_id(params[:user_id])

But I want it to also find by another param, project_id

How can I do something like this in Rails?

@permission = Permission.find_by_user_id_and_project_id(params[:user_id],params[:user_id])