Sometime ago I faced with NHibernate issue and spent much time on figuring out how to resolve it. Issue was with saving complicated entity.
Error:

The column name ‘RESOURCE_ID’ is specified more than once in the SET clause. A column cannot be assigned more than one value in the same SET clause. Modify the SET clause to make sure that a column is updated only once. If the SET clause updates columns of a view, then the column name ‘RESOURCE_ID’ may appear twice in the view definition.

With NHibernate Profiler I found that this fails on the next SQL: 

UPDATE SOME_TABLE SET SOME_COLUMN1 = ?, SOME_COLUMN2 = ?, RESOURCE_ID = ?, SOMETHING_ELSE = ?, RESOURCE_ID = ? WHERE SOME_TABLE_ID = ?

With better look you will see that there two times RESOURCE_ID mentioned so this looks like wrong mapping. But I was sure that everything is ok there.

This two References are wrong.
At first glance do you see why? At that moment I was not able also.

References(x => x.Resource)
        .Access.AsCamelCaseField(Prefix.Underscore)
        .WithForeignKey(“RESOURCE_ID”).TheColumnNameIs(“RESOURCE_ID”)
        .FetchType.Join();

References(x => x.ResourceRole)
        .Access.AsCamelCaseField(Prefix.Underscore)
        .WithColumns(“RESOURCE_ROLE_ID”, “RESOURCE_ID”)
        .FetchType.Join();

Here Resource has key (RESOURCE_ID).
ResourceRole has composite key (ROLE_ID and RESOURCE_ID).

After long searching and many tries I found why mapping is not correct. Even have my explanation for it.

When we reference ResourceRole we already use RESOURCE_ID field, so when we setup Resource and say “Hey, there is one more RESOURCE_ID“, then NHibernate cannot find out how to update all this correctly.

Solution:
insert=”false” update=”false” attributes for one of references solves the issue.

In Fluent NHibernate it looks like:

.SetAttribute(“update”, “false”);