Question

Suppose I have a class Parent, with two subclasses, AChild and BChild. I have these mapped to a single table using Entity Framework 5.0.0 on .NET 4.5, using TPH.

public abstract class Parent {
    public string Type { get; set; } // Column with this name in DB is discriminator.
    public string Status { get; set; }
}

public class AChild : Parent {
    // Other stuff.
}

public class BChild : Parent {
    // Other stuff.
}

The code to configure the mapping:

class ParentConfiguration : EntityTypeConfiguration<Parent> {
    Map((EntityMappingConfiguration<AChild> mapping) => mapping
        .Requires("Type")
        .HasValue("A"));
    Map((EntityMappingConfiguration<BChild> mapping) => mapping
        .Requires("Type")
        .HasValue("B"));
}

I have a need to run a query that returns both AChild and BChild objects. However, it needs to filter ONLY the AChild rows by a second column, which in this example I will call Status.

Ideally I would want to do the following:

public IList<Parent> RunQuery() {
    IQueryable<Parent> query =
        this.context.Set<Parent>()
        .Where((Parent parent) => !parent.Type.Equals("A") || parent.Status.Equals("Foo"))
        .OrderBy((Parent parent) => parent.Number);
    return query.ToList();
}

This doesn’t work. It insisted on looking for a “Type1” column instead of just letting both the discriminator and a “Type” property be mapped to the same “Type” column.

I know of the “OfType” extension method that can be used to completely filter down to one type, but that’s too broad a brush in this case.

I could possibly run multiple queries and combine the results, but the actual system I’m building is doing paging, so if I need to pull back 10 rows, it gets messy (and inefficient) to query since I’ll either end up pulling back too many rows, or not pull back enough and have to run extra queries.

Does anyone have any other thoughts?

Answer 1

There are few problems. First of all you cannot have discriminator mapped as a property. That is the reason why it is looking for Type1 column – your Type property results in second column because the first one is already mapped to .NET types of your classes. The only way to filter derived types is through OfType.

The query you want to build will be probably quite complex because you need to query for all Bs and concatenate them with result of query for filtered As. It will most probably not allow you to concatenate instances of Bs with As so you will have to convert them back to parent type.