Why is "Fixup" needed for Persistence Ignorant POCO's in EF 4?

Question

One of the much-anticipated features of Entity Framework 4 is the ability to use POCO (Plain Old CLR Objects) in a Persistence Ignorant manner (i.e. they don't "know" that they are being persisted with Entity Framework vs. some other mechanism).

I'm trying to wrap my head around why it's necessary to perform association fixups and use FixupCollection in my "plain" business object. That requirement seems to imply that the business object can't be completely ignorant of the persistence mechanism after all (in fact the word "fixup" sounds like something needs to be fixed/altered to work with the chosen persistence mechanism).

Specifically I'm referring to the Association Fixup region that's generated by the ADO.NET POCO Entity Generator, e.g.:

    #region Association Fixup

    private void FixupImportFile(ImportFile previousValue)
    {
        if (previousValue != null && previousValue.Participants.Contains(this))
        {
            previousValue.Participants.Remove(this);
        }

        if (ImportFile != null)
        {
            if (!ImportFile.Participants.Contains(this))
            {
                ImportFile.Participants.Add(this);
            }
            if (ImportFileId != ImportFile.Id)
            {
                ImportFileId = ImportFile.Id;
            }
        }
    }

    #endregion

as well as the use of FixupCollection. Other common persistence-ignorant ORMs don't have similar restrictions.

Is this due to fundamental design decisions in EF? Is some level of non-ignorance here to stay even in later versions of EF? Is there a clever way to hide this persistence dependency from the POCO developer?

How does this work out in practice, end-to-end? For example, I understand support was only recently added for ObservableCollection (which is needed for Silverlight and WPF). Are there gotchas in other software layers from the design requirements of EF-compatible POCO objects?

Answer 1

Found a few explanations - check them out!

POCO Template Code Generation Options (EF team blog)

Fixup

A fixup method is written for every navigation property on an entity and is called from the setter of the navigation property whenever its value changes. Its purpose is to ensure that each end of a bidirectional relationship stays in sync with the other. For example, in a one-to-many relationship between Cutomer and Order, whenever Order.Customer is set, the fixup method ensures that the Order is in the Customer’s Orders collection. It also keeps the corresponding foreign key property viz. Order.CustomerID in sync with the new Customer’s primary key (ID) value. This logic can be useful if the POCO entities are used independently of the EF stack, like for writing tests against them which don’t hit the database. Fixup ensures that the object graph is connected in the same way as you would expect while using them with EF. Fixup methods are a bit complex to write and hence it is useful to have them auto-generated if you are planning on using the entities in an EF independent scenario.

And also check out this POCO in the Entity Framework Part 1 which also has some sections on what fixups are and what they're needed for.