Problems adding a Child entity with a Parent entity

Question

We have two entities A and B. A is the parent and B is the child with an association defined.

We are attempting to save a new entity B. This new entity B contains an A entity. Entity A already exists in the database. When we attempt to save the new entity B, L2S first attempts to save entity A as a new entity before saving entity B. Of course, this results in a duplicate key error. My question is this. Why is L2S attempting to save entity A when the entity already exists in the database? It should know this because the primary key has a value.

How do I get around this?

Answer 1

Hi Randy,

 

Glad to see you again!

 

If first query the DataContext and get an existing Parent Table object, then set the Child Table’s navigation property to the existing Parent Table, the Parent Table object won’t be marked as Added, but Unchanged, since the DataContext has traced the object.  We don’t need to call InsertOnSubmit on the Child Table because the DataContext has know it is in Added state.  For detail, please see the following code snippet:

==========================================================
            using (DataClasses1DataContext db = new DataClasses1DataContext())

            {

                ChildTable c = new ChildTable()

                {

                    id = 10,

                    Name = "New Child"

                };

                ParentTable p = db.ParentTables.First();

                c.ParentTable = p;

                db.SubmitChanges();

            }
==========================================================

 

Another situation is that the Parent Table is not tracked by the DataContext.  We can new a Parent Table with the same primary key value which has existed in the database.  Or maybe the Parent Table object is passed in by another method.  In such a scenario, we can first attach the Parent Table to the DataContext and then add the Child Table object into the DataContext.  Please see the subsequent codes:

==========================================================
            using (DataClasses1DataContext db = new DataClasses1DataContext())

            {

                ChildTable c = new ChildTable()

                {

                    id = 10,

                    Name = "New Child"

                };

               

                ParentTable p = new ParentTable()

                {

                    id = 2,

                };

 

                c.ParentTable = p;

                db.ParentTables.Attach(p, false);

                db.ChildTables.InsertOnSubmit(c);

                db.SubmitChanges();

            }
==========================================================

In the above samples, the id columns are the PK of the Parent Table and Child Table. 

 

If you have any problems, please feel free to let me know.  

 

 

Have a great day!

 

 

Best Regards,
Lingzhi Sun

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Answer 2

Thanks Lingzhi. That's what I was looking for.

Randy