Why isn't my NHibernate bag collection setting the 'parent id' of the children dynamically?

Question

I have a new object with a collection of new objects within it on some property as an IList. I see through sql profiler two insert queries being executed.. one for the parent, which has the new guid id, and one for the child, however, the foreign-key on the child that references the parent, is an empty guid. Here is my mapping on the parent:

<id name="BackerId">
  <generator class="guid" />
</id>
<property name="Name" />
<property name="PostCardSizeId"  />
<property name="ItemNumber" />

<bag name="BackerEntries" table="BackerEntry" cascade="all" lazy="false" order-by="Priority">
  <key column="BackerId" />
  <one-to-many class="BackerEntry" />
</bag> 

On the Backer.cs class, I defined BackerEntries property as

IList<BackerEntry>

When I try to SaveOrUpdate the passed in entity I get the following results in sql profiler:

exec sp_executesql N'INSERT INTO Backer (Name, PostCardSizeId, ItemNumber, BackerId) VALUES (@p0, @p1, @p2, @p3)',N'@p0 nvarchar(3),@p1 uniqueidentifier,@p2 nvarchar(3),@p3 uniqueidentifier',@p0=N'qaa',@p1='BC95E7EB-5EE8-44B2-82FF30F5176684D',@p2=N'qaa',@p3='18FBF8CE-FD22-4D08-A3B1-63D6DFF426E5'

exec sp_executesql N'INSERT INTO BackerEntry (BackerId, BackerEntryTypeId, Name, Description, MaxLength, IsRequired, Priority, BackerEntryId) VALUES (@p0, @p1, @p2, @p3, @p4, @p5, @p6, @p7)',N'@p0 uniqueidentifier,@p1 uniqueidentifier,@p2 nvarchar(5),@p3 nvarchar(5),@p4 int,@p5 bit,@p6 int,@p7 uniqueidentifier',@p0='00000000-0000-0000-0000-000000000000',@p1='2C5BDD33-5DD3-42EC-AA0E-F1E548A5F6E4',@p2=N'qaadf',@p3=N'wasdf',@p4=0,@p5=1,@p6=0,@p7='FE9C4A35-6211-4E17-A75A-60CCB526F1CA'

As you can see, its not resetting the empty guid for BackerId on the child to the new real guid of the parent.

Finally, the exception throw is:

"NHibernate.Exceptions.GenericADOException: could not insert: [CB.ThePostcardCompany.MiddleTier.BackerEntry][SQL: INSERT INTO BackerEntry (BackerId, BackerEntryTypeId, Name, Description, MaxLength, IsRequired, Priority, BackerEntryId) VALUES (?, ?, ?, ?, ?, ?, ?, ?)] ---\u003e System.Data.SqlClient.SqlException: The INSERT statement conflicted with the FOREIGN KEY constraint

EDIT: SOLVED! The first answer below pointed me into the correct direction. I needed to add that back reference on the child mapping and class. This allowed it to work in a purely .net way - however, when accepting json, there was a disconnect so I had to come up with some quirky code to 're-attach' the children.

Answer 1

You may need to add NOT-NULL="true" to your mapping class:

<bag name="BackerEntries" table="BackerEntry" cascade="all" lazy="false" order-by="Priority">
  <key column="BackerId" not-null="true"/>
  <one-to-many class="BackerEntry" />
</bag>

as well as make sure that you have the reverse of the mapping defined for the child class:

<many-to-one name="parent" column="PARENT_ID" not-null="true"/>

I had similar issues with hibernate on my current project with parent-child relationships, and this was a part of the solution.