获得多对多关系的第一要素

Question

I am having some problems mapping one of my properties in nhibernate. I have a many-to-many relationship which is mapped to a collection. However, I also want to map a property to the first element of the many-to-many collection. Here is my code: The class I am mapping:

public class Project {
    public Project() {
        this.Addresses = new List<Address>();
    }

    public virtual int ID { get; set; }
    public virtual IList<Address> Addresses { get; set; }
    public virtual Address CurrentAddress { get; set; }
}

The classmap:

public class ProjectMap : ClassMap<Project> {
    public ProjectMap() {
        Table("Project");
        Id(x => x.ID).GeneratedBy.Identity();

        HasManyToMany(x => x.Addresses)
            .Table("ProjectAddress")
            .ParentKeyColumn("ProjectID")
            .ChildKeyColumn("AddressID").Cascade.All();
    }

But I need CurrentAddress to point to the first element of the Addresses property. So I tried to solve this with a getter that returns the first element like this:

        public virtual Address CurrentAddress {
        get {
            if (!Addresses.Any()) {
                Address newAddress = new Address();
                this.Addresses.Add(newAddress);
                return newAddress;
            }
            else {
                return this.Addresses.First();
            }
        }
    }

However, when I try to query over the Project class currentaddress, I get a QueryException: could not resolve property: CurrentAddress.

This is obviously because the CurrentAddress property is not mapped in Nhibernate, but how can I fix this?

Answer 1

As you correctly pointed out, you can't query non-mapped properties, since NHibernate has no way to generate the appropriate SQL - the property's getter can be arbitrarily complex.

There's no way to fix this, although you might have some luck exploring solutions like this: http://fabiomaulo.blogspot.cz/2010/07/nhibernate-linq-provider-extension.html

However, I suggest querying the non-mapped properties in memory.