C# – Fluent NHibernate cascade delete problem

cfluent-nhibernatenhibernate

I'm having a problem with fluent nhibernate cascade delete. I'm sure I'm doing something wrong because it isn't working.

Here are my objects:

public class Parent
{
    public int Id { get; set; }
    public IList<SequencedChild> SequencedChildren { get; set; }
}

public class SequencedChild 
{
    public int Id { get; set; }
    public int ParentId { get; set; }
    public int ChildId { get; set; }
    public int Sequence { get; set; }
}

public class Child 
{
    public int Id { get; set; }
}

And here is my mapping:

HasMany(m => m.SequencedChildren).Inverse().Cascade.Delete();

So I have a parent with some sequenced children and I want to update that parent to have no children. When I do an update with no sequenced children on that parent I expect that in my SequencedChild table the records that have the id of the parent will be deleted. But for some reason NHibernate is trying to update the ParentId of those records with null – which fails as ParentId is not null. EDIT: I'm also expecting that the Child object is unaffected (which is behaving correctly).

I had a look at a few questions and they all suggest the use of inverse but I am already doing this. What am I doing wrong?

Best Answer

So I managed to find a solution which turned out to have multiple steps:

As James pointed out in a comment ParentId/ChildId should be Parent/Child references not just the ids (+1 for that)
SequencedChild needs to have an explicit map that sets cascade to none
When doing the update, don't overwrite the SequencedChild list. First clear it then add the new items. (Or just clear if you arent replacing the items)
The Inverse() call is not necessary
The ParentId field in the db table should be nullable because nHibernate insists on updating it to null before it deletes it. (if anyone knows a way around this leave a comment)

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.

Fluent NHibernate: How to create one-to-many bidirectional mapping

To get a bidirectional association with a not-null foreign key column in the Details table you can add the suggested Owner property, a References(...).CanNotBeNull() mapping in the DetailsMap class, and make the Summary end inverse.

To avoid having two different foreign key columns for the two association directions, you can either specify the column names manually or name the properties in a way that gives the same column name for both directions. In this case you I suggest renaming the Details.Owner property to Details.Summary.

I made the Summary id generated by increment to avoid problems when inserting into the table since Summary currenty has no columns besides id.

Domain:

public class Detail
{
    public int id { get; protected set; }
    public string ItemName { get; set; }

    // Renamed to use same column name as specified in the mapping of Summary.Details
    public Summary Summary {get; set;} 
}

public class Summary
{
    public Summary()
    {
        Details = new List<Detail>();
    }

    public int id { get; protected set; }
    public IList<Detail> Details { get; protected set; }
}

Mapping:

public class DetailMap : ClassMap<Detail>
{
    public DetailMap()
    {
        Id(x => x.id)
            .GeneratedBy.Native();

        Map(x => x.ItemName)
            .CanNotBeNull();

        References<Summary>(x => x.Summary)
            // If you don't want to rename the property in Summary,
            // you can do this instead:
            // .TheColumnNameIs("Summary_id")
            .CanNotBeNull();
    }
}

public class SummaryMap : ClassMap<Summary>
{
    public SummaryMap()
    {
        Id(x => x.id)
            .GeneratedBy.Increment();

        HasMany<Detail>(x => x.Details)
            .IsInverse()
            .AsBag(); // Use bag instead of list to avoid index updating issues
    }
}

Best Answer

Related Solutions

How to delete child object in NHibernate

Fluent NHibernate: How to create one-to-many bidirectional mapping

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