Rails - Apr 2010 - Model question

I''m pondering a way of representing a scenario without too much
fiddling with the database. I''m 90% happy with it, but have one
problem (where I probably just don''t know the right AR approach to
use).

I have a "people" table, from which records are displayed depending on
whether they are new leads, exisiting customers, or just contacts
(maybe reps at stores). I''m keeping them in one DB table, but I
don''t
want to use STI, as a single person *might* be more than one "type".

Here''s the reasoning:

A Person can be flagged as "new lead".
A Person can have Orders (which makes them a customer).
A NewLead can have Orders - for instance trial packs that their sales
person has sent them - which makes them a customer as well as a lead.
A Person record can exist, and be neither a Customer or NewLead (I''m
calling these guys "contacts" for now, but they might as easily be
Foos or Bars...)

I want to be able to access the records using named_scopes, as, of
course, I want to be able to chain other scopes - so named_scopes are
essential (so I can get all the NewLeads for a given sales person, or
all the Contacts with a given surname, etc)

So I''ve got this structure:

class Person < ActiveRecord::Base
  # ...Person stuff
end

class NewLead < Person
  default_scope :conditions => "people.new_lead = 1" # ... simple
condition to return only new leads
end

class Customer < Person
  default_scope :group => "people.id", :joins => :orders  # ...
returns me unique instances (thanks to the :group) of just those
people who have any order records
end

class Contact < Person
  #... my problem... what does my default scope need to be to return
me people *without* orders, and with the "new_lead" flag set to zero?
end

The logic for NewLead and Customer is working perfectly - if I have a
Person with id=123 who has no orders, but new_lead set to "1", calling
"NewLead.find(123)" gives me a valid NewLead record, but
"Customer.find(123)" gives me a "record not found error" -
just what I
want to happen. Then if that person has an order placed for them, both
of the finds returns a valid record.

But how to return records with no Orders.... ? I''m thinking that I may
need something along the lines of:

  default_scope :conditions => "people.id not in (select distinct
people.id from people joins orders on orders.person_id = people.id)
and people.new_lead = 0"

But that seems ugly.

Any suggestions for neater ways?

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> But how to return records with no Orders.... ? I''m thinking that I
may
> need something along the lines of:
> But that seems ugly.
>
> Any suggestions for neater ways?
>
How about using counter caches - something like:

class Order < ActiveRecord::Base
  belongs_to :customer, :counter_cache => true
end

class Customer < Person
  has_many :orders
  default_scope :conditions => { :orders_count > 0}
end

http://guides.rails.info/association_basics.html#belongs-to-association-reference
(section 4.1.2.4)

Cheers,


Andy

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On 14 April 2010 12:32, Andy Jeffries
<andyjeffries-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> How about using counter caches - something like:
>
> class Customer < Person
>   default_scope :conditions => { :orders_count > 0}
> end
>
>
http://guides.rails.info/association_basics.html#belongs-to-association-reference
> (section 4.1.2.4)
> Cheers,
>
> Andy
Nice idea... that would do well, and clean it up a bit - I just tried
my approach (after fixing typos...) and it works, but just smells...
:-)

Thanks for that.

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