What exactly is "persistence ignorance"?
Persistence ignorance is typically defined as the ability to persist & retrieve standard .NET objects (or POCOs if you really insist on giving them a name). And a seemingly well accepted definition of a standard .NET object is:
"...ordinary classes where you focus on the business problem at hand without adding stuff for infrastructure-related reasons..."
However, I see people describing NHibernate as a framework that allows persistence ignorance, and yet it is a framework that cannot work on any standard .NET object, only standard .NET objects that adhere to particular design requirements, for example (source):
- All classes must have a default constructor
- Some features don't work unless classes are unsealed and all members are virtual
- Object identity doesn't work properly unless you abuse Equals/GetHashCode
(Aside: Before anybody gets upset, I don't mean to pick on NHibernate here, it's just a frequently quoted example of a framework that supposedly permits persistence ignorance. I'm sure similar arguments could be applied to other ORMs that claim the same.)
Now although the class in itself does not have any persistence-framework-specific attributes or base classes etc., to me it is not really "persistence ignorant" because it must follow a set of design guidelines to facilitate use by the chosen persistence framework. You must design and implement the class with the requirements of the persistence framework in mind; if you are ignorant of it the class may not work with it.
Where I'm having trouble with the definition of "persistence ignorance"/"POCO" is that I don't see how, conceptually, this is really any different to adding attributes such as [Serializable] or [DataContract] or [XmlType] or any other persistence-framework-specific annotations that facilitate the persistence and retrieval of the entity using that framework.
So, what exactly is "persistence ignorance"?
Clearly the definition of it as being able to persist "ordinary classes" is a fallacy because the NHibernate ones are only ordinary insofar as not referencing framework-specific classes, whereas they are extraordinary inasmuch as they require unusual design choices such as default constructors and all-virtual members and Equals/GetHashCode implementations on mutable types.
Is it therefore reasonable to say that "persistence ignorance" is true when objects facilitate the use of a persistence framework (either in design and structure or by use of framework-specific annotations) but do not perform any persistence logic themselves?
Answer
I would claim that, like most things, its a sliding scale. There are things that we make that want to have the property of persistence. On one end of the scale is this thing having all of the guts, dependencies, and code that is custom built to persist just this one thing in its particular way. On the other end of the scale is something that just magically happens, all without us doing much more than adding a token or setting a property somewhere that causes that thing to 'just persist'. In order to get to the magical side of the scale, there are frameworks, design guidelines, conventions, etc that assist the magic in happening. I think you could argue that a tool could be produced that had fewer requirements and restrictions than NHibernate but pursued the same goal; that hypothetical tool would be further along our scale.
I don't know that I like the term 'persistence ignorance' so much; its really about an object being ignorant of the implementation, the backing store, the cache, that sort of thing - an object is typically aware of whether or not it is persistent, though. But that's just semantics.