Fluent generates a column for a calculated property

[chester89]

This is situation to research, actually - courtesy of @robscottnh via email conversation. If you specify a formula to create a calculated property in a mapped class, fluent always adds a element to the mapping (even though it's a calculated property and there is no column in the table). If I generate the hbm.xml file and remove the tag, all is well, if I try to run with the tag in place nhibernate generates an error saying the column doesn't exist (actually it's the db that generates the error based on the generated sql).