c# - NHibernate: identifier of an instance of object was altered from x to y -

i have situation nhibernate. have piece of code:

private void unlinkproductsfrompriceagreement(namevaluecollection collection, iunitofwork unitofwork) {     // price agreement id     int priceagreementid = collection.getint("id");      foreach (int productid in collection.getids("unlinkproductslist"))     {         // product price         iproductprice productprice =             productpricerepository.find(p => p.priceagreement.id == priceagreementid && p.product.productid == productid).firstordefault();          // remove product price         productpricerepository.delete(productprice);     } } 

i exception when run code:

qe.common.core.exceptions.persistencyexception ---> nhibernate.hibernateexception: identifier of instance of qe.vending.core.domainobjects.product altered 210036 0    @ nhibernate.event.default.defaultflushentityeventlistener.checkid(object obj, ientitypersister persister, object id, entitymode entitymode) in d:\csharp\nh\nh\nhibernate\src\nhibernate\event\default\defaultflushentityeventlistener.cs:line 111    @ nhibernate.event.default.defaultflushentityeventlistener.getvalues(object entity, entityentry entry, entitymode entitymode, boolean mightbedirty, isessionimplementor session) in d:\csharp\nh\nh\nhibernate\src\nhibernate\event\default\defaultflushentityeventlistener.cs:line 84 

how possible?