I have three objects: level, product, part. Together they classify a
repair type.
- Replacing a part in a product has a level of difficulty associated
with it.
- The same part replaced in a different product has a different level
of difficulty.
How can I model an association among these?
I have created a join model, "Repair class," and connected all three
using :has_many through. However, this does not prevent multiple
records where the part and product stay the same, but the level
changes. In other words, I end up with multiple levels assigned to the
same part/product combination which does not happen in the real world.
I have three objects: level, product, part. Together they classify a
repair type.
- Replacing a part in a product has a level of difficulty associated
with it.
- The same part replaced in a different product has a different level
of difficulty.
How can I model an association among these?
I have created a join model, "Repair class," and connected all three
using :has_many through. However, this does not prevent multiple
records where the part and product stay the same, but the level
changes. In other words, I end up with multiple levels assigned to the
same part/product combination which does not happen in the real world.
You might want to describe exactly what join model you have created
Personally my instinct would be to have a unique index on the join
table enforcing that there is only one entry for each part/product pair
So I could create a unique index consisting of product_id, part_id. Is
it possible to then create a unique index consisting of level_id and
the other index? I am using MySQL currently.
I'm not sure I'm 100% clear, but it sounds like you are saying that a
Repair may only have one combination of Part/Product, and it just so
happens that this combination implies a Level. To me the validation
would be:
class Repair < ActiveRecord::Base
belongs_to :level
belongs_to :product
belongs_to :part
validates_presence_of :level_id, :product_id, :part_id
validates_uniqueness_of :part_id, :scope => [:product_id]
end
Assuming I understand the problem, I think your confusion stems from
the fact that the level isn't part of what defines a Repair, its
simply an attribute of the Repair. It is really the Part and the
Product that you should be concerned with.
Your last paragraph helps clarify it a little bit. I think you are
correct in that I am chasing the wrong thing. I think that stems from
not fully understanding the scope option of validates_uniqueness_of.
The documentation for it is not clear.
