has_many :through query question

rubyonrails-talk
1

I think this is obvious. but for some reason I'm not getting it.

Models: Bicycles, Accessories Join Model: Upgrades

The idea is that you can upgrade your bike by adding an accessory, and the upgrade price is often less than the accessory price would normally be. So the Upgrades table has id, bicycle_id, accessory_id, and price.

class Bicycle   has_many :upgrades   has_many :accessories, :through => :upgrades end

class Upgrade   belongs_to :bicycle   belongs_to :accessory

  # price defined in table end

Now my question is, I need to query the upgrades for a bicycle for a specific accessory.

I was doing this:

  accessory = Accessory.find_by_description("headlight")   bike.upgrades.find_by_accessory_id(accessory.id)

but it seems klunky. Is there a better way to perform this kind of lookup?

Thanks Jeff

2

It seems as bikes can have accessories that may or may not be upgrades, so:

bike.upgrades.accessories.find_by_description(“headlight”)

Otherwise, if there are only upgrade accessories, you can bypass a step:

bike.accessories.find_by_description(“headlight”)

Though in terms of good design (no dependency chaining), it might be a good idea to add a method to one of the models to hide this away. Say, Updgrade#find_accessory_by_description

Jason

3

Hi Jeff,

So if I am tracking with you here, you are looking for Accessories associated with a Bicycle as joined by the Upgrades. You should be able to just call bicycle.accessories.find(accessory) if you have an accessory, since Accessories are now associated with the Bicycle object via the has_many :though declaration. The has_many :through takes care of the need to go to the Upgrades object first from Bicycles since you specified the relationship in the model.

Hope this helps!

C

4

Almost but not quite - what I need is the Upgrade row, not the Accessory row, because I need to know the price for the upgrade given the desired accessory.

So bicycle.accessories.find_by_description("headlight") returns an Accessory object, but I still don't know the upgrade price.

That's why this is a bit tricky - I want an upgrade row, selected by the accessory description. Since the description is not in the join table, it seems I have to do two lookups - one to find the accessory, and the other to find the upgrade row given the accessory id (yuck).

Jeff

5

first toss the price and the id columns from your through table. you never need id and it might be screwing some thing up. the total order price should be calculated at run time and the accessory price should be in the accessories table.

I have a real join model here, not a simple habtm, because an upgrade price is different from the normal accessory price. Sorry if I wasn't clear about that.

(ID columns never "screw things up", btw).

def get_accessory(upgrade_type){ self.upgrades().each() do |up|    return up if up.description == "headlight" end

}this should cut down your proccessing to as it eliminates a costly DB query.(assuming there are not a huge amount of upgrades assigned to that bike)

Actually, self.upgrades will result in a query against the database... plus you've got another performance hit by iterating through the objects (I would suggest a .detect here instead of .each, btw).

Thanks anyway Jeff

6

It sounds like your model might need abstraction needs some work, although more information is needed to know what you are trying to accomplish. You might need to sit down and rethink the relationships between the models and what information should be in which table.

Why are you using an upgrades table? Are accessories available as non-upgrades? If an accessory is accessed as an upgrade is the information different than when as an accessory only? Is there a reason you don't want to put the upgrade fields into your accessory model and bypass the has_many :through association completely?

M<><