Entity Framework 4: Mastering AddObject and Attach

Effective use of Entity Framework hinges on understanding the distinct roles of ObjectSet.AddObject and ObjectSet.Attach. While AddObject inserts new entities, Attach manages existing ones. However, the situations requiring Attach can be nuanced.

One key use case for Attach involves entities detached from the context. This often occurs after retrieving an entity and subsequently closing the context. To re-engage this entity for modification, use Attach:

var existingPerson = new Person { Name = "Joe Bloggs" };
ctx.Persons.Attach(existingPerson);
existingPerson.Name = "Joe Briggs";
ctx.SaveChanges();

This generates an UPDATE statement, avoiding a redundant database retrieval.

Another valuable application of Attach is connecting existing, context-attached entities that lack automatic relationships. Consider a Person entity with an Addresses navigation property (a collection of Address entities). If you've loaded both Person and Address objects but their relationship isn't established, Attach provides the solution:

var existingPerson = ctx.Persons.SingleOrDefault(p => p.Name == "Joe Bloggs");
var myAddress = ctx.Addresses.First(a => a.PersonID != existingPerson.PersonID);
existingPerson.Addresses.Attach(myAddress);
ctx.SaveChanges();

Here, Attach updates the relationship without modifying the entities themselves.