Can EntityFramework support an EAV model?

Question

Can EntityFramework support an EAV model? Is this a workable scenario, or a nightmare? I want to use an EAV model for a system, and I'd like to embrace EF if possible, but I'm concerned that these two philosophies are in conflict.

Answer 1

It depends how do you expect to use EAV in the application. EF can be used to map this:

public partial class Entity
{
    // Key
    public virtual int Id { get; set; }
    // Other common properties 

    // Attributes
    public virtual ICollection<EavAttriubte> Attributes { get; set; }
}

// The simplest implementation
public class EavAttribute
{
    // Key
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual string Value { get; set; }
}

This is what can be persisted and what can be queried by Linq-to-entities. Now you can make your entity usable by defining helper properties (can be used only in your application but not by persistance or querying). These helper properties can be used only for well known attributes which will always exists for entity type - optional attributes must be still accessed in collection:

public partial class Entity
{
    // Just example without error handling
    public decimal Price
    {
        get
        {
            return Int32.Parse(Attributes.Single(a => a.Name == "Price"));
        }
        set
        {
            Attributes.Single(a => a.Name == "Price").Value = value.ToString();
        }
    }
}

This is not very nice because of conversions and collection searching. If you access data multiple times they will be executed multiple times.

I didn't tried it but I think this can be avoided by implementing a similar interface by each entity:

public interface IEavEntity
{
    // loads attribute values from Attributes collection to local fields
    // => conversion will be done only once
    void Initialize();
    // saves local values back to Attributes collection
    void Finalize();
}

Now you will handle ObjectMaterialized and SavingChanges events on ObjectContext. In the first handler you will execute Initialize if materialized object implements IEavEntity in the second handler you will iterate ObjectStateManager to get all updated or inserted entities implementing IEavEntity and you will execute Finalize. Something like:

public void OnMaterialized(object sender, ObjectMaterializedEventArgs e)
{
    var entity = e.Entity as IEavEntity;
    if (entity != null)
    {
        entity.Initialize();
    } 
}

public void SavingChanges(object sender, EventArgs e)
{
    var context = sender as ObjectContext;
    if (context != null)
    {
        foreach (var entry in context.ObjectStateManager.GetObjectStateEntries(
            EntityState.Added | EntityState.Modified))
        {
            if (!entry.IsRelationship)
            {
                var entity = entry.Entity as IEavEntity;
                if (entity != null)
                {
                    entity.Finalize();
                }
            }
        }
    }
}