NHibernate Mapping: Simple Join on a Foreign Key?

Join mappings are a PITA! The problem here is that Hibernate expects a join mapping to map from a Parent table to a Child table, whereas you're joining from the child table to the parent table.

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The Tables Currencies ---------- CurrencyId INT IDENTITY PK IsoCode4217 CHAR(3) -- "USD", "GBP", "EUR", etc Users ---------- UserId INT IDENTITY PK CurrencyId FK REFERENCES Currencies (CurrencyId) The Mapping The current application has a Currency object that needs the IsoCode4217 value. A Currency is not an entity in this application, it's its own weird hard-coded thing where I need to pass the IsoCode4217 value to a static function and get a Currency instance back. The database Currencies table is more of an enumeration table.

So I figured I'd just implement an IUserType and off I'll go, but this does not work as I expect: hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" class name="BlahBlah. UserProfile, BlahBlah. Model" schema="Core" table="Users" lazy="false" id name="Identity" access="field.

Camelcase" column="UserId" generator class="native" / /id join schema="Core" table="Currencies" fetch="join" key column="CurrencyId" property-ref="Currency" / property name="Currency" column="IsoCode4217" type="BlahBlah. CurrencyUserType, BlahBlah. Model" / /join /class /hibernate-mapping (I changed the angle brackets to regular brackets as I don't feel like thinking about what this little Markdown editor is going to do.

) The Ugly The SQL that I would expect is SELECT u. UserId, c. IsoCode4217 FROM Users AS you INNER JOIN Currencies AS c ON u.

CurrencyId = c. CurrencyId but instead NHibernate stubbornly gives me SELECT u. UserId, c.

IsoCode4217 FROM Users AS you INNER JOIN Currencies AS c ON u. UserId = c. CurrencyId -- DANG IT!

Is this really not supported, or am I overlooking something obvious? Is it possible to just "loop in" an extra column from another table for the purpose of a single entity's mapping? Thanks!

Nhibernate nhibernate-mapping link|improve this question asked Jul 9 '09 at 22:36Nicholas Piasecki8,2012143 100% accept rate.

– eyston Jul 10 '09 at 0:14 I'm using NHibernate-2.1.0. CR1. – Nicholas Piasecki Jul 10 '09 at 3:05.

Join mappings are a PITA! The problem here is that Hibernate expects a join mapping to map from a Parent table to a Child table, whereas you're joining from the child table to the parent table. Essentially, join mappings were designed to make it easy to join on tables that have a one-to-one relationship and where the parent table is joining to a child table.

I think in your situation you want to have a many-to-one mapping rather than a join.

Thanks, and yeah, that's what I thought. But would I be correct in assuming that I can't have a many-to-one mapping work with an IUserType? I really don't want Currency as a persistable entity, I want the three-letter code so I can call Currency.GetCurrency() on this third-party-provided Currency object.

My thinking is I'm going to have to wrap that Currency object with my own type. Blech. – Nicholas Piasecki Jul 9 '09 at 23:52.

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