Inserting parent and children with NHibernate at the same time

fluent-nhibernatenhibernate

I am attempting to save(insert) a Parent entity with list Child entities. Both entites use a Guid as primary keys. The keys are none nullable and there is no key relationship set up in the db.

The save doesn't work, an exception is thrown claiming – that I am trying to save a null into Parent foreign key in the Child table.

I was expecting nhibernate to create a key for the Parent and let its Child objects know about it. Is this a limitation of NHibernate or of using Guids as primary keys?

This answer suggests you have to set the parent manully, is this really the only way?

Here are my mappings for reference:

Parent mapping:

HasMany(x => x.Children).KeyColumn("ParentKey").Inverse().Cascade.All();

Child mapping:

 References(x => x.Parent).Not.Nullable().Column("ParentKey");

Best Answer

NHibernate is not magic. It is only an ORM, if your children do not have their reference set to the parent, why would it assume that because the parent has a list of children that the childrens' reference, in turn, SHOULD be a reference to the parent?

I believe you answered your own question when you stated that the Child entities don't have their Parent property populated (which means it is null, which means NHibernate would be attempting to save a 'null' value for the Parent id in your Child table).

If you were to use these objects WITHOUT NHibernate it would make sense that the Parent reference needs to be set on the Child entities when they are added.

EDIT: This is for the case where you have specified 'Inverse' on your mapping. If you were to remove this call to 'Inverse,' it should work the way you wanted it to work as Inverse states that the other end (the child entity) is responsible for keeping track of the relationship. This means you need to set the reference of the parent on the child manually.

However, removing the Inverse statement would cause the Child(ren) to be saved, the parent to be saved, then the Child(ren)'s parent id to be UPDATED. Since you have a null constraint on the parent id, this means it still won't work as it would initially insert the Child with a parent id of null.

The two solutions would be to either remove this constraint, or to just add a method to the Parent called AddChild:

public void AddChild(Child childObj)
{
    childObj.Parent = this;
    Children.Add(childObj);
}

Add another method called RemoveChild:

public void RemoveChild(Child childObj)
{
    if (Children.Contains(childObj))
    {
        Child.Parent = null;
        Children.Remove(childObj);
    }
}

Then, just use those methods for adding/removing children.

Related Solutions

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
    }
}

Fluent NHibernate Mapping is set to AllDeleteOrphan but is still trying to null the foreign key in the DB

Try setting .Inverse() on the ParentMap's HasMany, so it looks like:

HasMany(x => x.Children)
        .Not.LazyLoad()
        .KeyColumn("ParentId").Cascade.AllDeleteOrphan().Inverse()
        .Access.ReadOnlyPropertyThroughCamelCaseField(Prefix.Underscore);

Best Answer

Related Solutions

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

Fluent NHibernate Mapping is set to AllDeleteOrphan but is still trying to null the foreign key in the DB

Related Topic