Question

Supposed I have 4 tables defined as follows:

Projects: [ Project_Id, Project_Name ]

Vendors: [ Vendor_Id, Vendor_Name ]

Project_Vendors: [ Project_Vendor_Id, Vendor_Id, Project_Id ]

Payments: [ Payment_Id, Project_Vendor_Id, Payment_Amount ]

The relationships should be as follows:

• A Project can have many Vendors associated
• A Vendor can belong to many projects
• Many payments can be made to many vendors of a project (so what makes a payment unique is a project AND a vendor)

I can easily map Project and Vendor objects and their relation using the ProjectVendors lookup table with the following:

public class ProjectMapping : ClassMap<Project>
{
    public ProjectMapping()
    {
         Table("Projects");
         Id(x => x.Id).Column("Project_Id").GeneratedBy.TriggerIdentity();
         HasManyToMany(x => x.Vendors).Table("Project_Vendors")
             .ParentKeyColumn("Project_Id")
             .ChildKeyColumn("Vendor_Id")
             .Cascade.AllDeleteOrphan();
    }
}

My question is, if my classes are defined as follows:

public class Payment
{
    public virtual int? Id { get; set; }
    public virtual Project Project { get; set; }
    public virtual Vendor Vendor { get; set; }
    public virtual double Amount { get; set; }
}

public class Project
{
    public virtual int? Id { get; set; }
    public virtual string Name { get; set; }
    public virtual List<Vendor> Vendors { get; set; }
    public virtual List<Payment> VendorPayments { get; set; }
}

How can I map my Project and Payment objects using the lookup table Project_Vendors?

Answer 1

Not directly possible without mapping the Project_Vendors table because NHibernate has to know where the id to reference comes from. You might be able to fake it but i would advice against it.

class ProjectVendorPair
{
    public virtual int Id { get; set; } 
    public virtual Project Project { get; set; }
    public virtual Vendor Vendor { get; set; }
}

// in PaymentMap
References(x => x.Owner, "Project_Vendor_Id");

Or you change the database Structure:

• get rid of column Project_Vendor_Id
• have columns Project_Id and Vendor_Id in Payment table