How to do this query in NHibernate 3?

I have 4 tables and every one has a field named FormulaToken (nvarchar(250)).

When user enters new value in FormulaToken in any of the entity (connected to one开发者_JS百科 of these 4 tables), I need to check that this value is unique across all 4 tables.

How to do this?

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Found a solution. Instead of checking all 4 tables for the value of one field, I excracted this field to its own table and set it to unique indexed.