Rather than mapping the two columns as individual properties, what
you're probably better off doing is creating a custom IUserType
implementation to handle the mapping from the two columns into one one
date property (and vice-versa). See section 5.2.3: Custom value Types
in the NHibernate Documentation. 
http://nhforge.org/doc/nh/en/mapping.html#mapping-types-custom

Then, in your mapping (assuming you're doing fluent mapping, not auto
mapping), you'd add a call
to .CustomType(typeof(DateTimeInTwoColumnsUserType)) for your datetime
property.  Here's an example:
http://elegantcode.com/2009/11/01/state-pattern-enumeration-class-and-fluent-nhibernate-oh-my/

Speaking from experience, custom IUserTypes are not that difficult,
and really are your best friend when dealing with a legacy schema.

--
Will Green

On Aug 26, 11:50 pm, Klaus Lüdenscheidt <[email protected]>
wrote:
> i'm working with a legacy database which stores month and year of a date in 
> two string columns. LINQ to NHibernate doesn't support
> type conversion inside a query and strings can't be compared with <= or >=. 
> So i want to define a transient property in my entity
> class wich returns a DateTime from the two string columns. Nevertheless how i 
> define it (virtual, non virtual) i get a
> ConfigurationException from FluentNHibernate. When i defne a method which 
> returns the date LINQ To NHibernate throws an exception.
>
> Is there any way to tell FluentNhibernate not to map a property to a database 
> column?
>
> Regards
> Klaus

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