How do I create a navigation property that can navigate to more than one entity type? - entity-framework-5

Question

I have the following in my domain classes ( simplified )
public enum JobType
{
SalesOrder = 1,
StockOrder = 2
}
public class SalesOrder : LoggedEntity
{
public string Name { get; set; } // and other fields
}
public class StockOrder : LoggedEntity
{
public string Name { get; set; } // and other fields
}
public class Job : LoggedEntity
{
public int JobType { get; set; } // jobtype enum
public virtual LoggedEntity LinkedEntity { get; set; }
}
My context is as follows;
public class Context : DbContext
{
public DbSet<Job> Jobs { get; set; }
public DbSet<StockOrder> StockOrders { get; set; }
public DbSet<SalesOrder> SalesOrders { get; set; }
}
When I run the migration i get the error described [here][1] So using an abstract entity appears not to work.
My question was, how do I create a navigation property that can navigate to more than one entity type?
If JobType = SalesOrder then I want to navigate to sales order, if JobType = StockOrder then I want to navigate to stock order.
I wanted to use a Table Per Heirarchy Strategy [see TPH here][2]

Answer 1

The trick is to keep EF oblivious of the LoggedEntity class. Remodel your entities according to this example:
public enum JobType
{
SalesOrder = 1,
StockOrder = 2
}
public abstract class LoggedEntity
{
public int Id { get; set; }
public string Name { get; set; } // and other fields
}
public abstract class BaseOrder : LoggedEntity // New base class for orders!!
{ }
public class SalesOrder : BaseOrder
{ }
public class StockOrder : BaseOrder
{ }
public class Job : LoggedEntity
{
public JobType JobType { get; set; } // jobtype enum
public virtual BaseOrder Order { get; set; }
}
public class Tph2Context : DbContext
{
public DbSet<Job> Jobs { get; set; }
public DbSet<BaseOrder> Orders { get; set; }
}
You will see that the migration creates two tables, Jobs and BaseOrders (name to be improved). Job now has a property Order that can either be a SalesOrder or a StockOrder.
You can query specific Order types by
contex.Orders.OfType<StockOrder>()
And you will notice that EF doesn't know LoggedEntity, because
context.Set<LoggedEntity>()
will throw an exception
The entity type LoggedEntity is not part of the model for the current context.

Answer 2

how do I create a navigation property that can navigate
to more than one entity type?
You cannot do so. atleast not now. navigational properties are way of describing relationship between entities. at most, they represent, some sort of sql relationship. so you cannot alter or define such a relationship on the fly. you have to define it before hand.
Now in order to do that, you have to define separate navigational property for your separate conditions i.e.
public class Job : LoggedEntity
{
public int JobTypeSales { get; set; }
public int JobTypeStock { get; set; }
public virtual SalesOrder SalesOrder { get; set; }
public virtual StockOrder StockOrder { get; set; }
}
and then link them in configuration in modelbuilder through fluent API.
HasRequired(s => s.SalesOrder)
.WithMany()
.HasForeignKey(s => s.JobTypeSales).WillCascadeOnDelete(true);
HasRequired(s => s.StockOrder)
.WithMany()
.HasForeignKey(s => s.JobTypeStock).WillCascadeOnDelete(true);
and
as for your error "Sequence Contains No Elements"
this error comes, when the Linq query that you specified, is using either .First() or .Single(), or .ToList() and query returned no data.
so to avoid it use, .FirstOrDefault() or SingleOrDefault().
obviously with proper null check.