CODEWITHSUNDEEP
c#asp.netentity-frameworkmany-to-many
Aleksa
Aleksa

Reputation: 3104

Many to many relationship inserts

I have two classes, for instance:

class A {
    public virtual ICollection<B> BList { get; set; }
}

class B {
    public virtual ICollection<A> AList { get; set; }
}

Entity Framework then creates A and B tables and AB table that connects them.

In my application I get A_ID and B_ID, and I need to make connection between A and B.

What I did is fetch A by ID, then fetch B by ID, and then do A.BList.Add(BObject), but that is 3 database trips, 2 fetches + one insert.

Is there any way in Entity Framework to do it in one database trip?

UPDATE:

I looked at the link Martin provided, and wrote a simple example:

User user = new User { ID = 12135 };
Book book = new Book { ID = 1};

context.Users.Attach(user);
context.Books.Attach(book);

user.Books.Add(book);  

context.SaveChanges();

But I got null pointer exception at user.Books.Add(book); user.Books is null, and it throws exception when Add is called...

Upvotes: 1

Views: 133

Answers (2)

ocuenca
ocuenca

Reputation: 39326

You need to initialize your collection navigation properties in the entity's constructors:

public class A {
    public A()
    { 
       BList=new List<B>();
    }
    public virtual ICollection<B> BList { get; set; }
}

Do the same with the B entity.

Also you don't need to attach the new entities at the beginning, to save them you can do this:

  User user = new User { ID = 12135 };
  Book book = new Book { ID = 1};

  user.Books.Add(book);  
  context.Users.Add(user);
  context.SaveChanges();

That's going to insert the three rows in just one database trip

Upvotes: 4

Alexis Diel
Alexis Diel

Reputation: 1296

Maybe you can resume your code and try this:

User user = new User { 
                       ID = 12135 , 
                       Books = new ICollection<Book>() { new Book { ID = 1} } 
                     };

context.Users.Attach(user);

Or:

User user = new User { ID = 12135 };
Book book = new Book { ID = 1};

context.Users.Attach(user);
context.Books.Attach(book);

user.Books = new ICollection<Book>();

user.Books.Add(book);

To resolve null pointer expection need to add a initializer to Books user collection, in code or in the User class constructor.

Upvotes: 1

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