Relationships are more fun - part II

Trygve Reenskaug and James Coplien have just posted an article questioning the fundamentals of Object Orientation and the Model-View-Controller pattern.

It's interesting (if perhaps a little more wordy than necessary). And I don't agree with all of it (more of that later). But their characterisation of the relationship between the User's mental model and the Object model represented in software is spot on. But perhaps more interesting is their questioning of one of OO's central tenets: that all behaviour belongs in Objects (or Classes in programme language terms).

They assert that, whilst some behaviour does belong within the Object, much doesn't. To use their example: responsibility for increasing or decreasing an Account's balance properly lies with the Account. External parties can ask the Account to perform those operations, but it's up to the Account to do it. That's fully in line with the principles of Abstract Data Types on which OO is founded.

But what about performing a Transfer of funds from one Account to another? Where does that belong? Reenskaug and Coplien assert this doesn't belong with the Account Object - I agree. But that's how it would typically be handled in today's OO languages; so the Account class would typically have a method named "transferTo()", used as follows:

Account src;
Account dest;
src.transferTo(dest, 20.00);

What's wrong with that? From a mental model perspective, it doesn't work. When I think about transferring funds, I don't think about the responsibility lying with either the source or destination account. Sure they're both involved, but neither is responsible.

So where does it belong? In OOP terms we have nowhere else to put it other than in a Class. Reenskaug and Coplien suggest the notion of Interactions as first class entities; however their model still portrays an asymetric pattern (i.e. the responsibility for executing the transfer still lies with either the source or destination objects).

I prefer to think about it lying outside of either. But that doesn't mean the OO model is broken. Maybe it's just the way we use it today. Coming back again to the code example above:

src.transferTo(dest);

Now what happens if we look at this from a relationship perspective? What's the nature of the relationship? It's pretty obvious there's some link between the source and destination accounts - even if it's only transient. Here's an initial view as a UML class diagram?

Both source and destination are instances of Account, so the relationship would have to be recursive on Account. OK. Naming the ends? Easy; one is source, the other destination. Cardinality? Hmm, that's a bit more difficult. A given transfer has exactly one source and one destination account; and any account can be the source and/or target for many transfers. But we can't represent that on our relationship:

• 1:1 would only allow each account to be related to a single other: i.e. at most one transfer.
• many:many would only allow each account to be related to every other; but for every pair there could only be one transfer. Again, not what we need.

Let's revisit naming the relationship ends. I used "source" and "destination" above, which are role-based names. As I've stated previously, I prefer verb-based names because they bring out the "why" of the relationship. So how would we name the relationship ends using verb phrases?

Each Account may be the transfer source of one or more other Accounts
Each Account may be the transfer destination of one or more other Accounts

Kinda works, but it looks clumsy; it smells bad. So the cardinality doesn't work and the relationship naming is ugly. What does that tell us? Maybe there's no relationship? Well that's not true. It may be transient but both accounts are undeniably involved in something - otherwise there's no transfer.

Transfer. Hmm. What if, rather than looking at transfer as a verb, we treat it as a noun? What would that mean? Let's rework the model:

Let's examine the relationships again:

Each Transfer must debit exactly one Account
Each Account may be debited in one or more Transfers
An Account need not be debited in any Transfers

Each Transfer must credit exactly one Account
Each Account may be credited in one or more Transfers
An Account need not be credited in any Transfers

That's much better. The cardinality and phrasing capture exactly what we're trying to express - so the relationships look OK. But what about that "Transfer" class? Is it a proper class? Well, what does it mean to be a "proper" class? It's definitely an abstraction of behaviour - the transfer behaviour. What's more, it's a well encapsulated abstraction of behaviour. We are not loading the Account class with the responsibility for executing transfers; it need only respond to "credit" and "debit" requests: appropriate since it maintains the account balance.

What about state? That's OK too. The transfer value again properly belongs with the Transfer. We could also, if required, capture details of the time and success/failure of each transaction. Finally, we could, if required, track the state of each Transfer as it executes ("debiting source account", "crediting destination account", "completed successfully", ...).

There are critics of this approach who disagree with the idea of reification - treating behaviour as things. Rather, objects should be tangible things: Accounts, Customers, Chairs, Telephones, whatever - the things that are things in the real world.

The trouble is that doing so causes the situation that leads to a.transferTo(b). In fact, most of the interesting behaviour doesn't happen in the tangible objects. Transfer is an example of an Interaction object; one that coordinates a protocol amongst other objects. Interaction objects are useful and interesting precisely because they enable clean abstraction of behaviour amongst a number of other objects.

So is the OO model broken? No, not really. We just need to learn to use it properly. Going back to Coplien and Reenskaug, maybe it would help to promote Interactions to first class status, and hence increase focus on collaborations. Key to that is learning to love relationships - they're where all the fun happens.

Relationships are more fun

A number of years ago, I was fortunate enough to work for Steve Mellor's company. I remember vividly the first time I met him; I was very nervous. He was reviewing a domain model that I'd built, and I was amazed at how quickly he found a couple of logical flaws. One thing he said during the review has stuck with me ever since: "the fun is always in the relationships". It struck me as strange at the time; as someone schooled in OO, I had come to view Objects as the things that mattered. Relationships? Just pointers between Objects, right?

Wrong. Very wrong. Experience has fully vindicated Steve's assertion that the interesting stuff really does lie in the relationships, not the objects. In fact, the key to understanding any domain always comes down to formalising the relationships among the domain concepts. So I'm constantly frustrated by models in which relationships are second class citizens, demoted to pointers between objects with perhaps a role name. Something like this:
Why? Because it says nothing about the semantics of the relationship. Why does a Customer have a list of Accounts? Moreover, it says little about the domain - and doesn't prompt discussion or questions about the relationship. Can a customer really have more than one Account? How many Customers is each Account related to? What does the relationship mean when viewed from the Account's perspective anyway?

Steve (and my colleague, close friend and boss at the time Alistair Blair) instilled in me the importance of naming relationships using verb phrases. I've found it extremely valuable, more recently adopting the style recommended in Simsion and Witt's excellent book on Data Modelling. So the model above becomes:
With very little extra effort this provides much more information. We now know the relationship represents "Account Ownership". It doesn't cover being an authorised signatory, or a benefactor, or anything else. Are those things necessary? They may be. The point is the model now prompts that question.

The model also makes assertions about the relationship. An Account must be owned by exactly one Customer. Is that right? What about joint Accounts? Again, the model prompts the question. And so on. We can also generate textual facts (or "business rules") directly and algorithmically from the model, according to the technique described by Simsion and Witt. So we'd get:

Each Person may own one or more Accounts

Each Account must be owned by just one Customer

Furthermore, it tells us something about the behaviour, or interaction between Customer and Account. On first glance it's pretty simple: create a new account, assign it to the owner. Job done. But what happens if the Customer goes away? In typical OO programming terms, that responsibility would lie with the customer:

foreach a in self.accounts:
a.delete()

But why should it belong with the Customer? The required behaviour arises because of the Relationship: it's not intrinsic to the Customer. Another case in point is transfer of ownership. Where does that belong? The Customer? The Account? No. It belongs in the relationship.

So the fun does really happen in the Relationships - thanks Steve and Alistair for enlightening me.