Question

How to create a Multi-Column Index and/or Unique Constraint using NHibernate Mapping or Fluent NHibernate.

Answer 1

assign a index/unique constraint name to more then one property

<property name="A" index="AB" />
<property name="B" index="AB" />

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Theoretical it would also work with having more then one index on the same entity:

<property name="A" index="AB, ABC" />
<property name="B" index="AB, ABC" />
<property name="C" index="ABC" />

But there is a bug. I also wrote a patch. if you are interested in this, please vote for the bug or add comment or something.

Edit: just checked what happened to the bug. It is fixed in version 2.1.0, so it should perfectly work now. Many thanks to the great NHibernate developer team!