I have a following scenario--I have an advertiser object which has a collection of campaign objects.And,each campaign has collection of countries.
I create a new advertiser object,add 2 campaigns with countries to it.Now,i call save on advertiser object.Will nhibernate send only one sql statement to save all objects??Also,when i load parent object,all child object too get loaded--again with only one sql statement??

When you save the advertiser object, NHibernate would not automatically save the campaign object; unless some form of cascading is specified. Please read the documentation; specifically here:
http://www.hibernate.org/hib_docs/nhibe ... tiesvalues

NHibernate may or may not load your child objects depending on your mapping file. Please read the documentation; specifically here:
http://www.hibernate.org/hib_docs/nhibe ... e-fetching

thanks for the links,exactly what i needed.In our company,we are planning to use nettiers,which it seems does not give many features nhibernate gives