Forcing NHibernate to cascade delete before inserts

nhibernatenhibernate-cascadenhibernate-mapping

I have a parent object which has a one-to-many relationship with an ISet of child objects. The child objects have a unique constraint (PageNum and ContentID – the foreign key to the parent).

<set name="Pages" inverse="true" cascade="all-delete-orphan" access="field.camelcase-underscore">
    <key column="ContentId" />
    <one-to-many class="DeveloperFusion.Domain.Entities.ContentPage, DeveloperFusion.Domain, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
</set>

The problem I've hit is if I remove a ContentPage element from the parent collection, and then add a new one with the same unique key within the same transaction… You get a unique constraint violation because NHibernate attempts to perform the insert before the delete.

Is there a way to force NHibernate to perform the delete first?

Best Answer

There is no option to specify the order of operations in a transaction as it is hard-coded as follows (from the documentation):

The SQL statements are issued in the following order

all entity insertions, in the same order the corresponding objects were saved using ISession.Save()

all entity updates

all collection deletions

all collection element deletions, updates and insertions

all collection insertions

all entity deletions, in the same order the corresponding objects were deleted using ISession.Delete()

(An exception is that objects using native ID generation are inserted when they are saved.)

As such, can I challenge you to answer why you are adding a new entity with an existing identifier? An identifier is supposed to be unique to a specific "entity." If that entity is gone, so should be its identifier.

Another option would be to do an update on that record instead of a delete/insert. This keeps the ID the same so there is no unique constraint violation (on the key at least) and you can change all the other data so that it's a "new" record.

EDIT: So apparently I wasn't entirely paying attention to the question when I responded since this is a problem with a unique constraint on a non-primary-key column.

I think you have two solutions to choose from:

Call Session.Flush() after your delete which will execute all the changes to the session up to that point, after which you can continue with the rest (inserting your new object). This works inside of a transaction as well so you don't need to worry about atomicity.
Create a ReplacePage function which updates the existing entity with new data but keeps the primary-key and the unique column the same.

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

Related Topic