Question

This is a simplified example of the actual schema, so please bear with me. I have a table foo with an id and a value column. There’s also a bar table with a FK back to the foo table, and this table has an extended_value column. I want to have Foo’s class map left join Bar and try to use it’s extended_value column if present, otherwise fall back to Foo’s value.

Here’s the SQL equivalent of what I need:

select coalesce(b.extended_value, f.value) as value
  from foo f
  left join bar b on (b.id = f.id)

I’m using Fluent on top of NHibernate. This is what I’ve been attempting to use:

Join("bar", m =>
{
    m.Optional();
    m.KeyColumn("id");
    m.Map(foo => foo.Value).Formula("COALESCE(extended_value, value)");
});

But this is failing because the generated SQL is expecting both extended_value and value to be on the same table.

Any ideas?

Answer 1

My advice is to not do this in the mapping.

Map foo.value and bar.extended_value independently, and then use coalesce in your queries and/or create an unmapped property that uses the ?? operator.