Class design Help!

Rails
1

I have 3 classes (“Truck”, “Semi”, “Motorcycle”) that all inherit from
the “Vehicle” class.
Each subclass has an attribute for “Wheels” that tells how many wheels
the vehicle has (I do not store these values in the database because
they will never change)

I’m wondering what is the preferred method for handling this? Right now
I have individual methods in each class for “Wheels”:

For example:

class Truck
def wheels
4
end
end

class Motorcycle
def wheels
2
end
end

class Semi
def wheels
18
end
end

Is there a better way??? Thanks

2

Maybe you have a particular limited set of vehicles but trucks I know
have from 4 to 10 (or more) wheels and motorcycles have 2 or 3 wheels
and Semi-trailer trucks have 10 to 20 (or more) wheels depending on
application. I would store the number of wheels in the database.

Norm

3

Thanks for the Response Norm, That isn’t necessary in this instance
though (all trucks will have 4 wheels). Should I just keep with what I
have?

4

On Sat, May 25, 2013 at 11:21 PM, oliver bee [email protected]
wrote:

Thanks for the Response Norm, That isn’t necessary in this instance
though (all trucks will have 4 wheels). Should I just keep with what I
have?

I agree with Norm that this feels inauthentic - objects are typically
used to ‘model’ real world things and relationships, and I can see
lots of exceptions on your horizon :slight_smile:

In any case, I would probably do something like:

class Vehicle
attr_accessor :wheel_count
end

class Truck < Vehicle
def initialize(wheel_count = 4)
@wheel_count = wheel_count
end
end

… and so on.

I’d also recommend ‘Practical Object-Oriented Design in Ruby’
( http://www.poodr.info/ ) for some good thoughts on this.

FWIW,

Hassan S. ------------------------ [email protected]

twitter: @hassan

5

I originally tried to set it up that way and it works until I retrieve a
record from the database. I dont think it triggers the initialize
function. because I cant call the “wheels” method on an object after I
retrieve it.

6

Hassan S. wrote in post #1110193:

On Sat, May 25, 2013 at 11:21 PM, oliver bee [email protected]

Nice one!

class Vehicle
attr_accessor :wheel_count
end

class Truck < Vehicle
def initialize(wheel_count = 4)
@wheel_count = wheel_count
end
end

… and so on.

I’d also recommend ‘Practical Object-Oriented Design in Ruby’
( http://www.poodr.info/ ) for some good thoughts on this.

7

On 05/25/2013 11:21 PM, oliver bee wrote:

Thanks for the Response Norm, That isn’t necessary in this instance
though (all trucks will have 4 wheels). Should I just keep with what I
have?

What you suggested would work. My approach would be to have a vehicle
class/table with the characteristics of the vehicle types just because I
think STI is more bother than reward unless there are other things that
you do not mention that would drive you in that direction. I prefer the
simplest approach that will do what is needed. It makes it easier to
pass it on to someone else and I am not as likely to forget why things
are like they are.

Norm

8

On Sun, May 26, 2013 at 11:24 AM, oliver bee [email protected]
wrote:

I originally tried to set it up that way and it works until I retrieve a
record from the database.

Sorry, my bad for not explicitly saying that the approach above does
imply you’re going to persist that attribute (which makes sense if the
value is variable).

–
Hassan S. ------------------------ [email protected]

twitter: @hassan