EF5两级继承，TPT + TPH（代码优先）

Question

I've implemented a simple inheritance using EF5 code-first TPT. My base class is "Person" and my inherited class is "User". Tables are named accordingly.

Due to the fact my software is gonna be reused as a framework for other people to build on top of it, I want to give them an easy tool to expand the software without changing the database. I need only to fetch the right class type, I won't need to update it. In order to achieve this I want a specialized layer for Users with TPH. Developers will add their class in code and insert their record marking the type.

I've added to Users table the "Discriminator" field but now I'm getting this error trying to load the model:

Error 3032: Problem in mapping fragments : mapped to the same rows in table

It's not really clear what the error means.. Can anyone suggest an explanation/solution for it?

Thanks in advance

Answer 1

After some digging, I found out that it's possible indeed, I'm not sure since which version but it works like a charm on EF5.

The solution to the above error is to Map the relation manually with Fluent API. The TPT layer requires:

• An inheritance relationship between two classes
• The same primary key on both tables, and for the inherited type table a foreign key on the primary key.

Here is the class definitions:

[Table("Persons", Schema="MySchema")]
public partial class Person
{
    public int PersonId { get; set; }
    public string Name { get; set; }
}

[Table("Users", Schema = "MySchema")]
partial class User : Person
{
}

The TPH layer doesn't seem to work just adding the "Discriminator" field. The solution I found is:

• Adding a "UserType" field to the database.
• Map it using Fluent API.

An important note to make is that the "UserType" field cannot be included in class definition or you'll get the error above.

public class CustomUser : User
{
}

Inside the DbContext class, on the OnModelCreating override:

modelBuilder.Entity<User>().Map<User>(m =>
    {
        m.ToTable("Users");
        m.Requires("UserType").HasValue("User");
    }).Map<CustomUser>(m =>
    {
        m.Requires("UserType").HasValue("CustomUser");
    });

Finally, having this kind of code in DbContext is not really reusable, so I've moved it inside a EntityTypeConfiguration class as following:

public class CustomUserConfiguration : EntityTypeConfiguration<CustomUser>
{
    public CustomUserConfiguration()
    {
        Map<CustomUser>(m => m.Requires("UserType").HasValue("CustomUser"));
    }
}

The DbContext can now use a little of reflection to load all the EntityTypeConfiguration classes

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    MethodInfo addMethod = typeof(ConfigurationRegistrar).GetMethods().Single(m => m.Name == "Add" && m.GetGenericArguments().Any(a => a.Name == "TEntityType"));
    IList<Assembly> assemblies = AppDomain.CurrentDomain.GetAssemblies().Where(a => !a.GetName().Name.StartsWith("System") && !a.GetName().Name.StartsWith("Microsoft")).ToList();
    foreach (Assembly assembly in assemblies)
    {
        IList<Type> types = assembly.GetTypes().Where(t => t.BaseType != null && t.BaseType.IsGenericType && t.BaseType.GetGenericTypeDefinition() == typeof(EntityTypeConfiguration<>)).ToList();
        foreach (Type type in types)
        {
            Type entityType = type.BaseType.GetGenericArguments().Single();
            object entityConfig = assembly.CreateInstance(type.FullName);
            addMethod.MakeGenericMethod(entityType).Invoke(modelBuilder.Configurations, new object[] { entityConfig });
        }
    }
}