Well, if you need a list, you need an index. Therefore, I would recommend just doing this ICollection if the order is not significant.
The display should look something like this:
HasMany(x=> x.userRights).Element("RightID").AsBag();
However, looking at your tables, I noticed something strange. You are trying to use one-to-many without having a primary key in the User_Rights table. If you got User_Id in UserRights, this should work.
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