Help with model relationship for Product/Bill of Materials

Rails
1

I am struggling to find the best way to set up relationships to
achieve this so that it fits the rails idioms.

I have a table of products and I want to be able to create a bill of
materials for a product…

So a product can either be a single item, or a package consisting of
several other products.

I have created a table:

class BomItem < ActiveRecord::Base

belongs_to :product

belongs_to :component,
:class_name=>‘Product’,
:foreign_key=>‘component_id’

validates_numericality_of :quantity

So for a product p, I can have:
bom=p.bom_items
item=bom[0] etc

and for each item I can have:
c=item.component (which is a product)

Now for c I would like to be able to ask
c.component_of

at the moment, I can only think to set up a product method

Perhaps that is the best way to do it, but I just feel I am not
modeling this in the best way. I also want to make sure a product
cannot have a bom and be a component in the same bom - since that is
circular

If any experts in data modeling have any suggestions or advice I’d be
grateful

ps. I did have one bright idea to sub class product into component
and packet, which worked really well from console tests, but messed up
my Product views because although @product=Product.find id, the url
for product_path(@product) becomes packet path if it has type=‘Packet’
and of course there is no such route. That did my head in so I
dropped the idea.

Tonypm

2

Tony,
I’m currently working on the same thing but have decided to make BOM’s
a separate entity and not an item in the item file. Thus I have an
Assembly table that has many Assyitems. These Assytitems hold foreign
keys to the Item table. This works well when users don’t have to do
nested BOM’s.
Kathleen

3

Keeping your BomItem model pretty much as is you could have

class Product …
has_many :bom_items
has_many :components, :through => :bom_items
end

This gives you what you’ve already got.

Then you can say

has_many :bom_uses, :foreign_key => ‘component_id’, :class_name =>
‘BomItem’
has_many :uses, :through => :bom_uses, :source => :product

(please think of better names than I have :slight_smile:
Then given a production, p.components are the products it is made of,
and p.uses is the products that uses it as a component

Fred

4

Kathleen,

thanks for the suggestion. I thought about pulling BOM out to a
separate model that would be used to reference the BOM Items. I went
backwards and forwards a few times and decided to try it with just the
bom_items and see if I was losing any functionality. Still putting
together the view to allow BOMs to be created for a product, I will
see how it works out.

Thanks
Tony