Unfortunately, no. There's _lots_ of differences. There are a number of attributes that they share, but they each have completely different business logic for deriving those values, and they each have attributes that are unique to themselves that would make no sense to have on the other.

As uncommon as it is to actually need inheritance in the Model, the amount of work I would have to do to not use it would far exceed the amount of work I have to do to use it.

Believe me, I'm not using it just because it is a cool OO concept. Not that I don't do that in other places in my app - just not here. :-)

Dave

On Apr 7, 2008, at 3:18 PM, Alexander Spohr wrote:

What are the other differences between RawMaterial and ManufacturedPart? Wouldn’t it be enough to just flag the RawMaterial ones as such and just have one class for both?

        atze


Am 07.04.2008 um 18:02 schrieb David Avendasora:
You know, I was hoping to avoid the whole question if the domain was actually well-modeled or not, but with such dispersions cast on my flawless modeling skills, I must respond! :-P

I have 2 types of Parts. One purchased from an outside vendor (RawMaterial), and ones that are manufactured (ManufacturedPart). A ManufacturedPart is made up of any number of component Parts. These component Parts can be _either_ a RawMaterial or another ManufacturedPart.

So I have modeled it like this:

ManufacturedPart ->> BillOfMaterial ->> BOMComponent -> Part

To make things more complicated each given Manufactured Part can have one or more ways of making it and therefor have multiple BillsOfMaterial.

Hence, I have a branching, recursive relationship tree that can be any number of levels deep:

ManufacturedPart ->> BillOfMaterial ->> BOMComponent -> ManufacturedPart ->> BillOfMaterial ->> BOMComponent -> ManufacturedPart ->> Rinse, lather, repeat.

Or it can be very simple

ManufacturedPart ->> BillOfMaterial ->> BOMComponent ->> RawMaterial

What all this comes down to is that billsOfMaterial() does _not_ belong on Part as it does not belong on RawMaterial because a RawMaterial is NOT manufactured by us so it will never have a BillOfMaterial.

But there are several places in my code that I want to recursively navigate this tree and I just don't see a "right" way to do that without some variation of instanceOf (in code or in a fetch), a case statement or adding the method to Part.

The simplicity of one simple method returning NSArray.emptyArray wins. Even though it sullies my object graph, it does so in a much less complicated, fragile way.

Dave


On Apr 7, 2008, at 11:26 AM, Chuck Hill wrote:

The real question is whether it makes sense in the domain for a Part to have a bill of materials. The default implementation could be

public NSArray billOfMaterials() { return this; }

With complex parts overriding it as appropriate. It could just be that the model / design is incomplete and leading David into doing Bad Things in code.

Otherwise, (a) the model is just wrong, or (b) David's processing is just wrong.


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