Insertion order of multiple records in Entity Fram

问题:

I'm having trouble with EF reordering my inserts when I try and add an entity with multiple children all at once. I've got a 3 level structure with one-to-many relationships between each (Outer 1--* Item 1--* SubItem). If I try and insert a new Outer with Items and Subitems, the Items which contain SubItems end up being inserted first.

Sample Code (.NET 4.5, EF 5.0.0-rc):

public class Outer
{
    public int OuterId { get; set; }
    public virtual IList<Item> Items { get; set; }
}

public class Item
{
    public int OuterId { get; set; }
    [ForeignKey("OuterId")]
    public virtual Outer Outer { get; set; }

    public int ItemId { get; set; }
    public int Number { get; set; }

    public virtual IList<SubItem> SubItems { get; set; }
}

public class SubItem
{
    public int SubItemId { get; set; }

    [ForeignKey("ItemId")]
    public virtual Item Item { get; set; }
    public int ItemId { get; set; }
}

public class MyContext : DbContext
{
    public DbSet<Outer> Outers { get; set; }
    public DbSet<Item> Items { get; set; }
    public DbSet<SubItem> SubItems { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        Database.SetInitializer(new DropCreateDatabaseAlways<MyContext>());
        MyContext context = new MyContext();

        // Add an Outer object, with 3 Items, the middle one having a subitem
        Outer outer1 = new Outer { Items = new List<Item>() };
        context.Outers.Add(outer1);
        outer1.Items.Add(new Item { Number = 1, SubItems = new List<SubItem>() });
        outer1.Items.Add(new Item { Number = 2, SubItems = new List<SubItem>(new SubItem[] { new SubItem() }) });
        outer1.Items.Add(new Item { Number = 3, SubItems = new List<SubItem>() });

        context.SaveChanges();

        // Print the order these have ended up in
        foreach (Item item in context.Items)
        {
            Console.WriteLine("{0}\t{1}", item.ItemId, item.Number);
        }
        // Produces output:
        // 1       2
        // 2       1
        // 3       3
    }
}

I'm aware of this answer by Alex James which states that inserts may need to be reordered in order to satisfy relational constraints, but that is not the issue here. His answer also mentions that they can't track the order of items in order-preserving structures such as Lists.

What I'd like to know is how I can get these inserts to be ordered. While I can rely on sorting my inserted items by a field other than the PK, it's a lot more efficient if I can rely on the PK order. I don't really want to have to use multiple SaveChanges calls to accomplish this.

I'm using EF5 RC, but judging by the other unanswered questions around, this has been around for some time!

回答1:

What I'd like to know is how I can get these inserts to be ordered.

You cannot. Order of database commands is EF's internal behavior. If you want to control the order of commands don't use tools which abstract you from low level database interactions - use SQL directly.

Edit based on comment:

Yes it is low level interaction because you are putting expectations on the order of SQL commands when working with abstraction you don't have under your control. At high level you are getting something different because you are using expectations which don't work with that abstraction. If you want to have control over order of SQL commands you must either force EF by saving items one by one (=> multiple SaveChanges and TransactionScope) or write SQL yourselves. Otherwise use separate column for ordering.

Btw. EF doesn't save the entity as you see it. It has its own change tracker holding references to all your attached instances. References are held in multiple Dictionary instances and dictionary doesn't preserve insertion order. If these collections are used for generating SQL commands (and I guess they are) no order can be guaranteed.

回答2:

Tables in the database are sets. That means that the order is not guaranteed. I assume in your example that you want the results ordered by "Number". If that is what you want, what are you going to do if that number changes and it doesn't reflect the order in the database anymore?

If you really want to have the rows inserted in a specific order, multiple SaveChanges are your best bet.

The reason nobody wants to call SaveChanges multiple times is because this feels exactly how it is: a dirty hack.

Since a primary key is a technical concept, it shouldn't make any functional sense to order your results on this key anyway. You can order the results by a specific field and use a database index for this. You probably won't see the difference in speed.

Making the ordering explicit has other benefits as well: it is easier to understand for people who have to maintain it. Otherwise that person has to know that ordering on primary key is important and gives the correct results, because in an other (completely) unrelated section of your application, it accidentally is the same order as the number field.