Insert a new row into a table ith nhibernate when the primary key is an identity column

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NHibernate

exceptions

Hi
I have been pulling my hair out on this for a couple of hours at least now.

So I am using nhibernate to map my db to my code. In the DB I have table called "Product" which has a column called "ProductId" which is the primary key and also marked as an identity column in sql server 2012.

My class' mapping with the Id columned defined as:
Id(x => x.ProductId, mapper => mapper.UnsavedValue(0));

However after creating a new Product object and trying to save it I get the following error and inner exception:
{"could not execute batch command.[SQL: SQL not available]"}

Cannot insert explicit value for identity column in table 'Products' when IDENTITY_INSERT is set to OFF.

The exception is pretty clear and the product Id is set to 0. How do I tell nHibernate to not supply the Id column when creating a new object in the database?

Any help greatly appreciated

thanks