NHibernate WARNING and the data is not saved - nhibernate

Question

While I am saving by calling SaveOrUpdate(), I got this warning and the data is not saving in the database after calling Transaction.Commit().
NHibernate.Engine.ForeignKeys - Unable
to determine if [project name] with
assigned identifier [primarykey] is
transient or detached; querying the
database. Use explicit Save() or
Update() in session to prevent this.
I am inserting a new object. Google search tell me to call Save() instead of SaveOrUpdate(): means Save() is only for inserting.
I search in Google and do not see much about this.
Could anyone give me suggestion for this problem or this warning?
Edit:
Here is the simulated sample mapping files -
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly=""
namespace="">
<class name="Customer" table="[dbo].[Customer]" optimistic-lock="none" >
<id name="CustomerId" column="CustomerId" >
<generator class="assigned"/>
</id>
<property name="Name" column="Name" />
<property name="Age" column="Age" />
<set name="CustomerDetails" cascade="none" inverse="true" fetch="select">
<key>
<column name="CustomerId"/>
</key>
<one-to-many class="CustomerDetail"/>
</set>
<many-to-one name="MGender" fetch="select" cascade="none">
<column name="GenderCode"/>
</many-to-one>
</class>
</hibernate-mapping>
<class name="CustomerDetails" table="[dbo].[CustomerDetail]" optimistic-lock="none" >
<id name="CustomerDetailId" column="CustomerDetailId" >
<generator class="assigned"/>
</id>
<property name="Detail1" column="Detail1" />
<many-to-one name="Customer" fetch="select" cascade="none">
<column name="CustomerId"/>
</many-to-one>
</class>
<class name="MGender" table="[dbo].[MGender]" optimistic-lock="none" >
<id name="GenderCode" column="GenderCode" >
<generator class="assigned"/>
</id>
<property name="Description" column="Description" />
<set name="Customers" cascade="none" inverse="true" fetch="select">
<key>
<column name="GenderCode"/>
</key>
<one-to-many class="Customer"/>
</set>
</class>

Answer 1

You're using an assigned identifier so you need to set the unsaved-value attribute so that NHibernate can determine if an entity should be inserted or updated. Or you can explicitly call Save for new entities.
<id name="CustomerId" column="CustomerId" unsaved-value="???" >
<generator class="assigned"/>
</id>

Answer 2

Please note that if you are using a custom id, ie not using Id(x => x.Id).GeneratedBy.Assigned() and you do not have any version or timestamp columns, there is no way NHibernate would know if thuis is a new row or an existing one. So, I fixed my problem by using GeneratedBy.UuidString(). Works like a charm. Almost put my code into production with this error message, fortunately my information was being saved.