Because testing NHibernate mapping between objects, and database, is something everyone who is using NHibernate has to do. NHibernate community provides a ready helper that will just do that for us.
Introducing PersistenceSpecification, our hero.
As an example code
Let us have the following class

public class Vendor
    public virtual string AccountNumber { get; set; }
    public virtual string Name { get; set; }
    public virtual Int16 CreditRating { get; set; }
    public virtual bool Active { get; set; }
    public virtual bool PreferredVendorStatus { get; set; }

And we have a mapping for it to the database.
In order to test the mapping with PersistenceSpecification, we write the following test:

public void Vendor_Test()
    var vendor = new Vendor { Name = "Any name", CreditRating = 3, 
        Active = true, PreferredVendorStatus = true, AccountNumber = "Acc Number" };
    new PersistenceSpecification<vendor>(session)
        .CheckProperty(p => p.AccountNumber, vendor.AccountNumber)
        .CheckProperty(p => p.Active, vendor.Active)
        .CheckProperty(p => p.CreditRating, vendor.CreditRating)
        .CheckProperty(p => p.Name, vendor.Name)
        .CheckProperty(p => p.PreferredVendorStatus, vendor.PreferredVendorStatus)

And that is it.