join across databases with nhibernate

I am trying to join two tables that reside in two different databases. Every time, I try to join I get the following error:

An association from the table xxx refers to an unmapped class开发者_StackOverflow社区. 

If the tables are in the same database, then everything works fine.

I am not sure about this error. But I have also faced problems in joining tables from different databases in a procedure. I created temp table in one of the databases and then inserted data from the other table and did join using temp table with table in the same database.