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.

Answer 2

I had this problem and it took me forever to figure out. The Child table has to allow nulls on it's parent foreign key. NHibernate likes to save the children with NULL in the foreign key column and then go back and update with the correct ParentId.