Unhandled Exception after Upgrading to Entity Framework 4.3.1

Question

Error:

Unhandled Exception: System.Data.SqlClient.SqlException: The operation failed because an index or statistics with name \'IX_ID\' already exi

Answer 1

In my opinion this is clearly a bug.

The problem starts with the observation that EF creates an index IX_ID at all. If you strip down the model to the following...

public abstract class Lesson
{
    public Guid ID { get; set; }
}

public class RecurringLesson : Lesson
{
}

public class MyContext : DbContext
{
    public DbSet Lessons { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity().ToTable("RecurringLessons");
    }
}

... and let EF create the database schema you get two tables Lessons and RecurringLessons as expected for a TPT inheritance mapping. But I am wondering why it creates two indices for the table RecurringLessons:

• Index PK_RecurringLessons (clustered, unique) with Index column ID
• Index IX_ID (not clustered, not unique) with Index column ID again

I don't know if there is any benefit for the database to have a second index on the same column. But for my understanding it doesn't make sense 1) to create an index on the same column that is already covered in the PK clustered index, and 2) to create a not unique index on a column which is the primary key and therefore necessarily unique.

Moreover due to the one-to-one relationship EF tries to create an index on the table of the dependent of this association which is PrivateMakeUpLessons. (It's the dependent (and not the principal) because Cancellation is required in entity MakeUpLesson.)

ID is the foreign key in this association (and primary key at the same time because one-to-one relationships are always shared primary key associations in Entity Framework). EF apparently always creates a index on the foreign key of a relationship. But for one-to-many relationships this is not a problem because the FK column is different from the PK column. Not so for one-to-one relatonships: The FK and PK are the same (that is ID), hence EF tries to create an index IX_ID for this one-to-one relationship which already exists due to the TPT inheritance mapping (which leads to a one-to-one relationship as well from database perspective).

The same consideration as above applies here: The table PrivateMakeUpLessons has a clustered PK index on column ID. Why is a second index IX_ID on the same column required at all?

In addition EF doesn't seem to check that it already wants to create an Index with name IX_ID for the TPT inheritance, leading finally to the exception in the database when the DDL is sent to create the database schema.

EF 4.2 (and before) didn't create any indices (except PK indices) at all, this was introduced in EF 4.3, especially indices for FK columns.

I didn't find a workaround. In the worst case you have to create the database schema manually and avoid that EF tries to create it (= disable database initialization). In the best case there is a way to disable automatic FK index creation, but I don't know if it's possible.

You can submit a bug report here: http://connect.microsoft.com/VisualStudio

Or maybe someone from EF development team will see your question here and provide a solution.