How do you map strings in the database to enums in your model without introducing a second property? - entity-framework

Question

My database has a table like this:
Cats
- CatId INT PK
- Name VARCHAR(100)
- FavoriteToy VARCHAR(100)
And my code looks like this:
Cat.cs
public int CatId { get; set; }
public string Name { get; set; }
public Toy FavoriteToy {get; set; }
StaticVariables.cs
public enum Toy { Box, Ball, StuffedAnimal }
In a normalized database I would use a lookup table in the database to store all the toys and then the Cats table would just store a ToyId. But for this situation it's a lot easier to just store the FavoriteToy as a string even though it will be redundant.
The problem is I don't know how to convert a string in the database to an enum in code without creating a second FavoriteToyString property and having FavoriteToy just be a computed that returns the enum derived from FavoriteToyString.
I've heard this might be possible in the current version of entity framework. Is that true? Can you please show me how to do this?

Answer 1

You may use DTO class and automapper to solve your issue :)

Answer 2

Generally, yes a lookup table reference is a better option since your data can comply with referential integrity. That is, No cat records with toys that your Enum hopefully doesn't contain. (Though your Enum would need to be kept in sync with the Toys table.) You can configure EF to store enumerations as a string using a bit of a trick with the mapping:
public class Cat
{
[Key]
public int CatId { get; set; }
public string Name { get; set; }
[Column("FavoriteToy")]
public string FavoriteToyMapped { get; set; }
[NotMapped]
public Toy FavoriteToy
{
get { return (Toy)Enum.Parse(typeof(Toy), FavoriteToyMapped); }
set { FavoriteToyMapped = value.ToString(); }
}
}
The caveat of this approach is that where you might use Linq to Entity to filter on your cat's favorite toy, you need to reference the FavoriteToyMapped value in the query expression because EF/DB won't know what FavoriteToy is.
I.e.
Cats with a favorite toy of "Yarn"
var catsThatLoveYarn = context.Cats.Where(c => c.FavoriteToyMapped == Toys.Yarn.ToString()).ToList();
// not
var catsThatLoveYarn = context.Cats.Where(c => c.FavoriteToy == Toys.Yarn).ToList();
// Will error because EF doesn't map that property.
Once you are working with instances of entities, that the set of entities has been pulled back from the database, you can further access/refine queries with FavoriteToy. Just be cautious and prepared for the unknown field if you use it too early and EF goes and tries to compose a query.
var threeYearOldCats = context.Cats.Where(c => c.Age == 3).ToList();
var threeYearOldCatsThatLoveYarn = threeYearOldCats.Where(c => c.FavoriteToy == Toys.Yarn).ToList();
This is Ok because the .ToList() in the first query executed the EF-to-SQL, so threeYearOldCats is now a local List<Cat> of cat entities, not an IQueryable<Cat>.