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I have a class that inherits a base class to which another class has relationships.

Example:

  • Base class: Animal
  • Subclass 1: Dog
  • Subclass 2: Cat
  • Related one-to-many table: Vaccinations
    • A dog can have multiple vaccinations. This is implemented as a List<Vaccination>.
    • A cat can have multiple vaccinations.
    • A vaccination record can only have one animal associated with it.
    • A vaccination doesn't know if it's associated with a dog or a cat. (Dogs and cats use non-colliding GUIDs.)

There is no Animal table; Animal is an abstract class. But Vaccination knows only about Animal, not about Dog. (EF, however, knows about both.) I have my class libraries split such that Animal and Vaccination are in a core library and Dog is in another library that references the core library.

When using Entity Framework Code First, the Vaccinations table is getting the extra column: Dog_ID, as Dog class's List<Vaccination> explicit declaration is creating an inference. As such, this column maps the vaccination record to the dog. This would be fine, except for the fact that I want to share this extra column across multiple types of Animal. So for example rather than have a Dog_ID and a Cat_ID I'd like to have an Animal_ID that could join to either the Dog or the Cat.

As Animal is abstract and has no DB table, can I accomplish this with perhaps a fluent statement and/or property/attribute declarations?

Jon Davis
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3 Answers3

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Your Vaccination table will always have additional FK column for each vaccinated type of animals unless you create Animal table and make relation to that table. EF cannot map and high level abstraction of relations - relations must follow same rules as if you create them directly in the database.

If only some animals can be vaccinated you can add another table to hierarchy and make relation with that new table (EF is not able to work with interfaces). You will need Table per Type mapping for that and it will make performance of your queries much worse in current EF version.

Ladislav Mrnka
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  • "relations must follow same rules as if you create them directly in the database." Technically, I can relate one column as a virtual foreign key to multiple tables in SQL Server. I was hoping I could do this in EF CF. – Jon Davis Jul 05 '11 at 08:29
  • Technically you can relate one column in multiple relationship but it would mean that Vacation has AnimalId related to Dog, Cat and whatever else and once you fill that column every "parent" table must have record with the same Id = your vacation will be always related to dog, cat, ... – Ladislav Mrnka Jul 05 '11 at 08:36
  • Check the link with TPT mapping I provided in the answer. And Animal can be still abstract. – Ladislav Mrnka Jul 05 '11 at 08:55
  • Not working for me, unfortunately. I moved Vaccinations list to Animal and DbSet to context. EF creates the Animal table. But if I call context.Animals.Add(dog) I get an error (type mismatch), so I use context.Set.Add(dog), and now Animals table is empty and Dogs table is full and meanwhile Vaccinations still has a Dog_ID column. I'm sure I'm doing something wrong but that article seems too vague as it only demonstrates populating a DB via a navigation property (User -> BillingData) whereas the data I am storing (Animal/Dog) is not nested. – Jon Davis Jul 05 '11 at 10:09
2

Assuming all your animals will have vaccinations

public abstract class Animal
{
   public string ID { get; set; }
   public ICollection<Vaccination> Vaccinations { get; set; }
}

public class Vaccination
{
   public string ID { get; set; }
   public string AnimalID { get; set; }
   public virtual Animal Animal { get; set; }
   //other properties
}

Then you can inherit Cat and Dog and use Table per Type mapping.

Eranga
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  • Thank you. This would be fine, but unfortunately only a few types of Animal can have vaccinations. I have an interface (i.e. ICanHaveVaccinations) that declares support for it on the inherited types that need it but not on the base class. Maybe it will suffice to declare AnimalID and Animal on Vaccination, though... I'll give that a try. :) – Jon Davis Jul 05 '11 at 07:56
  • @stimpy77 then declare the `Vaccinations` property on `ICanHaveVaccinations` implementations. (eg: Cat, Dog) – Eranga Jul 05 '11 at 08:00
  • Also, putting Animal on Vaccination I get an error about it being abstract (that is, not having a known derived type) and won't continue to build the DB. – Jon Davis Jul 05 '11 at 08:23
1

So as I implemented what is currently marked as the answer (that I ultimately need to go ahead and create that Animal table) I ran into problems because, as I explained in my question, I have an isolated project declaring the "Dog", and for this and related issues I am directed here, here, and here.

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Jon Davis
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