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So I have two tables Users and Groups. These tables are (in the database) liked with a UGlink link table. Now except for the primary-foreign keys the link table have an extra column: Date.

From what I understand this means that I have to have two Many-to-One, with the link "in the middle".

However since I'm almost never is interested of the extra column value, is there anyway to avoid the link? That is I want to be able to write:

thisUser.Groups

to get the groups, instead of: 

thisUser.UGlinks.Group

enter image description here

Markus
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  • *"However since I'm almost never interested of the extra column value"*, either you need to account for it or you don't, which is it? – Matthew Mar 21 '14 at 15:38
  • I will need it some times. And then you would agree with Radims answer? @Matthew – Markus Mar 24 '14 at 11:14

1 Answers1

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The many-to-many, without the explicit mapping of the pairing table as an entity - is in NHibernate of course suported. So, in case, that the Date column is autogenerated, or nullable (does not have to be inserted by app/NHiberante), we can do it like here: 6.8. Bidirectional Associations

<class name="User">
    <id name="Id" column="Uid"/>
    ...
    <bag name="Groups" table="UGlink" lazy="true">
        <key column="Uid"/>
        <many-to-many class="Group" column="Gid"/>
    </bag>
</class>

<class name="Group">
    <id name="id" column="Gid"/>
    ...

    <!-- inverse end -->
    <bag name="Users" table="UGlink" inverse="true" lazy="true">
        <key column="Gid"/>
        <many-to-many class="User" column="Uid"/>
    </bag>
</class>

So, what we have is a mapping, in which NHiberante does care about the pairing table, and we can do:

thisUser.Groups

But if I could suggest, do not go with many-to-many. The many-to-one with pairing object is (I'd say) better solution, because it will support searching Users by Groups and vice versa.

See Chapter 24. Best Practices, cite:

Don't use exotic association mappings.

Good usecases for a real many-to-many associations are rare. Most of the time you need additional information stored in the "link table". In this case, it is much better to use two one-to-many associations to an intermediate link class. In fact, we think that most associations are one-to-many and many-to-one, you should be careful when using any other association style and ask yourself if it is really neccessary.

Here is some more detailed explanation how to do it without many-to-many: Nhibernate: How to represent Many-To-Many relationships with One-to-Many relationships?

Community
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Radim Köhler
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    I disagree with the guidance that "Good use cases for a real many-to-many associations are rare." They occur frequently when linking an entity to value or lookup objects. – Jamie Ide Mar 21 '14 at 18:10
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    My experience is different - having pairing table mapped as entity brings only advantages. Mostly, you can use the pairing object for *subqueries* and search for `Users` while filtering `Groups`. In fact, the amount of mapping is so small when avoiding the `many-to-many` ... but/and the benefit at the end is priceless... Just my experience, anyway – Radim Köhler Mar 21 '14 at 18:12
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    heads up the best practices link has moved: http://nhibernate.info/doc/nh/en/index.html#example-mappings-authorwork – mcfea Mar 09 '15 at 20:29