C# – NHibernate parent mapping does not create child foreign-key

cnetnhibernate

I am just learning NHibernate. I have been using examples from the documentation and here at stackoverflow, but I must be missing something.

I have a PARENT object that has a collection of CHILD. A CHILD is meaningless without a PARENT, so the database has FK set to NOT NULL. Implementing NHibernate from CHILD to PARENT works fine, though I have no need for this direction of relationship.

Instead, I tried to implement the PARENT owning the relationship, but I consistently get a Database error bubbling up "Cannot insert the value NULL into PARENT_ID". Neither the ID nor the entity of the PARENT is being stored in the CHILD when saving.

See the code sample below. Please advise.

Class Files

public class PARENT {
   private readonly IList<CHILD> _children = new List<CHILD>();
   public virtual Id { get; set; }
   public virtual void AddChild(CHILD child) {
      _children.add(child);
   }
}
public class CHILD {
   public virtual Id { get; set; }
}

Mappings

<class name="PARENT" table="Parent">
  <cache usage="read-write"/>

  <id name="Id" column="Id" unsaved-value="0" >
    <generator class="identity" />
  </id>

  <bag name="Children" access="field.camelcase-underscore" cascade="all-delete-orphan">
    <key column="ParentId"/>
    <one-to-many class="CHILD"/>
  </bag>

</class>
<class name="CHILD" table="Child">
  <cache usage="read-write"/>

  <id name="Id" column="Id" unsaved-value="0" >
    <generator class="identity"/>
  </id>

</class>

Best Answer

You must define both sides of the relationship in your mapping and object model. You then declare one as inverse="true" in the mapping. So something like this should work:

Class files

public class PARENT {
   private readonly IList<CHILD> _children = new List<CHILD>();
   public virtual Id { get; set; }
   public virtual void AddChild(CHILD child) {
      _children.add(child);
   }
}
public class CHILD {
   public virtual PARENT Parent { get; set; }
   public virtual Id { get; set; }
}

Mappings

<class name="PARENT" table="Parent">
  <cache usage="read-write"/>

  <id name="Id" column="Id" unsaved-value="0" >
    <generator class="identity" />
  </id>

  <bag name="Children" access="field.camelcase-underscore"
     cascade="all-delete-orphan" inverse="true">
    <key column="ParentId"/>
    <one-to-many class="CHILD"/>
  </bag>

</class>
<class name="CHILD" table="Child">
  <cache usage="read-write"/>

  <id name="Id" column="Id" unsaved-value="0" >
    <generator class="identity"/>
  </id>
  <many-to-one name="Parent" class="PARENT" column="ParentId" />

</class>

And you might want to change your AddChild method to this:

public virtual void AddChild(CHILD child) {
   _children.add(child);
   child.Parent = this;
}

Related Solutions

How to delete child object in NHibernate

You are getting the first error because, when you remove the items from the collection, NHibernate's default mode of operation is to simply break the association. In the database, NHibernate tries to set the foreign key column on the child row to null. Since you do not allow nulls in that column, SQL Server raises the error. Clearing the collection will not necessarily delete the child object, but one way to do so is to set cascade=all-delete-orphan. This informs NHibernate that it should delete the newly orphaned rows instead of setting the foreign key column.

You are getting the second error because when you call SaveOrUpdate NHibernate first deletes all of the child objects. Then, because neither relationship is marked as inverse, NHibernate also tries to set the foreign key column in your child table to null. Since the rows have already been deleted, you receive the second error. You need to set inverse=true on one side of your relationship to fix this. This is usually done on the one (primary key or parent) side. If you do not do this, NHibernate will make the appropriate updates for each side of the relationship. Unfortunately, running two updates is not the appropriate thing to do.

You should always mark one side of your relationships as the inverse side. Depending on how you code, you may or may not need to use cascading. If you want to take advantage of one shot deletes as you are trying to do using Clear(), you need to define your cascade.

NHibernate sets Foreign Key in secondary update rather than on initial insert violating Not-Null Constrain on key column

A few problems with your mapping... You have a bidirectional relationship and NHibernate needs to know which way to update it. In the OO world, references only go one way and there is no way for NHibernate to know that Parent->Children is the same FK as Child->Parent. Right now you have Child->Parent set to ReadOnly(). This is telling NHibernate not to update this property. So it tries to insert the Child (with a null parent) and then update the FK from the Parent side. This isn't working if you have a not null constraint on your FK. The usual way to map this is to use Inverse=true on the parent side and let the child worry about persistence. (It's your job in the OO model to ensure that the Parent->Children collection contains the same set of references as the set of Child->Parent relationships.)

public class ParentMap : ClassMap<Parent>
{
    public ParentMap()
    {
        Id(x => x.ParentId).GeneratedBy.Native();
        Map(x => x.Name);
        HasMany(x => x.Children).KeyColumn("ParentId").Inverse().Cascade.SaveUpdate();
    }
}

public class ChildMap : ClassMap<Child>
{
    public ChildMap()
    {
        Id(x => x.ChildId).GeneratedBy.Native();
        Map(x => x.Name);
        References(x => x.Parent).Column("ParentId").Not.Nullable();  // Removed ReadOnly()
    }
}

The SQL statements sent to the database when saving are now:

INSERT INTO [Parent]
           (Name)
VALUES     ('P1' /* @p0 */)
select SCOPE_IDENTITY()

INSERT INTO [Child]
           (Name,
            ParentId)
VALUES     ('C1' /* @p0 */,
            1 /* @p1 */)
select SCOPE_IDENTITY()

Best Answer

Related Solutions

How to delete child object in NHibernate

NHibernate sets Foreign Key in secondary update rather than on initial insert violating Not-Null Constrain on key column

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