Entity Framework 5 - Enum based Discriminator for

问题:

I have the following (abbreviated for clarity) - an enum, a base class with that enum, and two derived classes that set the enum to a specific value.

public enum MyEnum
{ 
    Value1, Value2
}

public class MyBaseClass
{ 
    public MyEnum { get; protected set; }
}

public class DerivedOne: MyBaseClass
{
    public DerivedOne { MyEnum = MyEnum.Value1; } 
}

public class DerivedTwo: MyBaseClass
{
    public DerivedTwo { MyEnum = MyEnum.Value2; }
}

What I want to do, is have Entity Framework 5 automatically distinguish between DerivedOne and DerivedTwo, with a MyEnum value based discriminator. I should be able to do this as, by convention, every MyEnum == MyEnum.Value1 represents DerivedOne, and MyEnum == MyEnum.Value2 represents DerivedTwo.

I tried this in my DbContext:

public class MyDbContext : DbContext
{
    DbSet<MyBaseClass> MyBaseClass { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<MyBaseClass>()
                    .Map<DerivedOne>(m => m.Requires(x => x.MyEnum == MyEnum.Value1));

        base.OnModelCreating(modelBuilder);
    }
}

However, this throws the following InvalidOperationException:

The expression 'x => (Convert(x.MyEnum) == 0)' is not a valid property expression. The expression should represent a property (...)

Edit: I believe I got a little farther using this:

modelBuilder.Entity<MyBaseClass>().Map<DerivedOne>(m => m.Requires("MyEnum")
                                  .HasValue((Int32)MyEnum.Value1));

Now I get this EntityCommandCompilationException:

Problem in mapping fragments starting at line (...) Condition member 'MyBaseClass.MyEnum' with a condition other than 'IsNull=False' is mapped. Either remove the condition on MyBaseClass.MyEnum or remove it from the mapping.

Any hints on how I can solve this? Thanks!

回答1:

As far as I know you cannot do that. Doing the explicit Requires to specify the disciminator is only to give it a name - not to connect it to your property.

As far as I know that always results in that error (later) that you're describing. If you want to specify discriminator it has to be 'automatic'one (at least I never managed to define it that way)

But you don't need that really. The 'enum' and discriminator is built into the type you get back - based on the discriminator values, EF/CF is constructing either 'Base` or 'DerivedOne' or DerivedTwo.

So to implement what you want you can do the following...

public class MyBaseClass
{
    [NotMapped()]
    public virtual MyEnum MyEnum { get { return MyEnum.Base; } }
}

public class DerivedOne: MyBaseClass
{
    public string OneProp { get; set; }
    public override MyEnum MyEnum { get { return MyEnum.One; } }
}

public class DerivedTwo: MyBaseClass
{
    public string TwoProp { get; set; }
    public override MyEnum MyEnum { get { return MyEnum.Two; } }
}

Or just use is instead (if it works for you)...

if (entity is MyBaseClass) // instead of enum  

or Query by...

.OfType<MyBaseClass>();

回答2:

As of EF 6.1, I was in fact able to use an enum as a discriminator column, in spite of this error:

Additional information: Values of type 'MyEnum' cannot be used as type discriminator values. Supported types include byte, signed byte, bool, int16, int32, int64, and string.

All I have to do was something like this:

public enum MyEnum
{ 
    Value1, Value2
}

public class MyBaseClass
{ 
    public MyEnum { get; protected set; }
}

public class DerivedOne: MyBaseClass
{
    public DerivedOne()
    {
        MyEnum = MyEnum.Value1;
    } 
}

public class DerivedTwo: MyBaseClass
{
    public DerivedTwo()
    {
        MyEnum = MyEnum.Value2;
    }
}

public class MyDbContext : DbContext
{
    DbSet<MyBaseClass> MyBaseClass { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Configurations
            .Add(new DerivedOneConfiguration())
            .Add(new DerivedTwoConfiguration());
    }
}

public class DerivedOneConfiguration : EntityTypeConfiguration<DerivedOne>
{
    public DerivedOneConfiguration()
    {
        Map<DerivedOne>(_ => _.Requires("MyEnum").HasValue((int)MyEnum.Value1).IsRequired());
    }
}

public class DerivedTwoConfiguration : EntityTypeConfiguration<DerivedTwo>
{
    public DerivedTwoConfiguration()
    {
        Map<DerivedTwo>(_ => _.Requires("MyEnum").HasValue((int)MyEnum.Value2).IsRequired());
    }
}

So the secret was using (int)MyEnum.Value* instead of MyEnum.Value*...

回答3:

Based on the answer of @rsenna but updated with mapping based on Microsofts Fluent Api original documentation.

https://msdn.microsoft.com/en-us/library/jj591617%28v=vs.113%29.aspx?f=255&MSPPError=-2147217396

public enum MyEnum
{ 
    Value1, Value2
}

public class MyBaseClass
{ 
    [NotMapped]
    public MyEnum MyEnum { get; protected set; }
}

public class DerivedOne: MyBaseClass
{
    public DerivedOne()
    {
        MyEnum = MyEnum.Value1;
    } 
}

public class DerivedTwo: MyBaseClass
{
    public DerivedTwo()
    {
        MyEnum = MyEnum.Value2;
    }
}

public class MyDbContext : DbContext
{
    DbSet<MyBaseClass> MyBaseClass { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<MyBaseClass>()
            .Map<DerivedOne>(x => x.Requires("MyEnum").HasValue((int)MyEnum.Value1))
            .Map<DerivedTwo>(x => x.Requires("MyEnum").HasValue((int)MyEnum.Value2));
    }
}

回答4:

I wonder if adding a third value to MyEnum to represent the base class will help. And then set MyBaseClass.MyEnum to that particular 'default' enum value in the constructor.

I think the Table-per-heirarchy structure needs that EVERY type must have a valid discriminator. So, you have 3 types:

1. MyBaseClass
2. DerivedOne
3. DerivedTwo

Even if your application won't use MyBaseClass in its base form ever, EF still needs a valid discriminator mapping for it.