Question

NHibernate really does not seem to like returning a ReadOnlyCollection, depsite me having implemented what I’ve read in about 30 places as the correct access strategy for a read only collection backed to a private field.

I have the following code in my entity:

private readonly IList<TagAlias> _aliases = new List<TagAlias>();

public IEnumerable<TagAlias> Aliases
{
    get
    {
        return new ReadOnlyCollection<TagAlias>(this._aliases); 
    }
}

and the following mapping to allow access to the backing field

public sealed class TagMap : ClassMap<Tag>
{
    public TagMap()
    {
        Table("Tag");
        Id(x => x.Id).Column("TagId").GeneratedBy.Identity();
        Map(x => x.Value).Column("TagName");
        HasMany(x => x.Aliases)
            .AsSet()
            .Access.ReadOnlyPropertyThroughCamelCaseField(Prefix.Underscore)
            .KeyColumn("TagId")
            .LazyLoad()
            .Inverse()
            .Cascade.AllDeleteOrphan();
    }
}

Why on earth does NHibernate still insist on getting the backing list via the ready only collection when I do something like .Clear(), rather than accessing it like I have told it to in the mapping? I am loath to change my domain model just for the sake of the persistence layer, but NHibernate just doesn’t seem to be wanting to cooperate.

The error I get is “A collection with cascade=”all-delete-orphan” was no longer referenced by the owning entity instance” but it goes away when I just return this._aliases in the property getter.

Answer 1

I generally use collections like this and don’t have problems:

private IList<OrderLine> orderLines;
public virtual IEnumerable<OrderLine> OrderLines 
{ 
    get { return orderLines.Select(x => x); } 
}

HasMany(x => x.OrderLines)
            .Access.CamelCaseField()
            .KeyColumn("ORDER_ID")
            .Inverse()
            .Cascade.AllDeleteOrphan();

I’m not sure of exactly how you are using your collections to get this error but I generally have add and remove methods for my collections inside the classes. If this doesn’t help you maybe you could post the example that causes that error.

Here is a related article I posted several months back:

Exposing HasMany and ManyToMany relationships as IEnumerable