Problem while working with many-to-many relationships in Entity Framework

Question

I create some items from a class X.

I add them to de base, do SaveChanges and all this...

The class Y has a relationship many-to-many with X.

Using another Context, I create a Y instance, putting into the collection of X the elements I've created.

I add Y to Y entity set, it is fine.

When I do Context.SaveChanges(), I get:

A value shared across entities or associations is generated in more than one location. Check that mapping does not split an EntityKey to multiple server-generated columns.

Have you ever seen this error?

EDIT: at the beginning, I've put 1-to-many, after I've noticed it is in fact many-to-many.

EDIT 2: showing the way this is being done. Unlike many people while using .net, we do use layers (business, data...). This is the test case:

[TestMethod]
public void WorksWithAreaCategories()
{
    using (new TransactionScope())
    {
     //arrange
     var context = ContextFactory.Create();
     var categoryBusiness = new CategoryBusiness(context);
     var category = new Category
       {
        Name = "TestCategory###"
       };
     categoryBusiness.Add(category);

     var areaBusiness = new AreaBusiness(context);
     var area = new Area
       {
        Name = "TestArea###",
        Description = "TestAreaDescription###",
        Categories = new List<Category> {category}
       };

     //act
     areaBusiness.Add(area);

     //assert
     var areaFromDb = areaBusiness.FindById(area.AreaID);
     Assert.IsNotNull(areaFromDb.Categories);
     Assert.IsTrue(areaFromDb.Categories.Count > 0);
     Assert.IsTrue(areaFromDb.Categories.Any(c => c.CategoryID == category.CategoryID));
    }
}

They share the same context. The business layers call SaveChanges in the end of each Add.

Answer 1

When the error says "more than one location", it really means more than one context. Data "belongs" in a specific context and you get problems if you try to move it between contexts.

There are 2 ways to fix it:

• Do everything in the same context
• When you create the Y instance, read the X elements from disk, so that they are in the same context, before adding the X elements to Y.

EDIT based on comment:

Try using a single save changes.

Answer 2

How I resolved it:

After adding both of them, I established the relationship with Update.

[TestMethod]
public void WorksWithAreaCategories()
{
    using (new TransactionScope())
    {
     //arrange
     var context = ContextFactory.Create();
     var categoryBusiness = new CategoryBusiness(context);
     var category = new Category
         {
          Name = "TestCategory###"
         };
     categoryBusiness.Add(category);

     var areaBusiness = new AreaBusiness(context);
     var area = new Area
         {
          Name = "TestArea###",
          Description = "TestAreaDescription###",
         };
     areaBusiness.Add(area);

     //act
     area.Categories = new List<Category> { category };
     areaBusiness.Update(area);

     //assert
     var areaFromDb = areaBusiness.FindById(area.AreaID);
     Assert.IsNotNull(areaFromDb.Categories);
     Assert.IsTrue(areaFromDb.Categories.Count > 0);
     Assert.IsTrue(areaFromDb.Categories.Any(c => c.CategoryID == category.CategoryID));
    }
}