Using Model.find for inner joins and to 'dedupe' a result se

Hi,

I’m using rails 1.2.3 and mysql 5

I have a collection of models like this:

Car has_many Passengers;
Passenger has_many Pocket_Items

I want to issue a Car.find command from rails where I find all cars
where at least 1 passenger is carrying a knife (or whatever) in his/
her pocket.

The details are shown below:

Table 1: cars (id, license_plate, make, model)
Table 2: passengers (id, car_id, first_name, last_name)
Table 3: pocket_items (id, passenger_id, item_description)

The description of the Car model would be something like this:

class Car < ActiveRecord::Base
has_many :passengers
has_many :pocket_items, :through => :passengers
end

so, e.g. car # 1 may contain 2 passengers. passenger 1 may have a
wallet and a knife, whereas passenger 2 may have a knife and keys.

So, now let’s say that I want to find all cars where at least 1
passenger is carrying a knife.

in SQL, I might issue the following statement

select
c.id,
c.license_plate,
c.make,
c.model
from cars c
inner join passengers p on (p.car_id = c.id)
inner join pocket_items pi on (pi.passenger_id = p.id)
where pi.item_description = ‘Knife’

this works, except that if a car contains 2 people, each carrying a
knife, the same car will be returned on 2 separate rows.

obviously, this can be eliminated by changing the statement above to
“select distinct …”

My questions are:

  1. Is there another (more elegant) way of demanding an INNER JOIN from
    rails aside from issuing it directly from the :joins option within the
    find method?

  2. Is there a rails idiom for ‘deduping’ my result set (i.e. I only
    want a result set of UNIQUE cars from my find query. From the agile
    development book, ed 2 (page 341), one can ‘dedupe’ child objects
    using the modifier :uniq => true. However, i’m not sure if there’s a
    way to do this on the parent object.

  3. are there any (more efficient) approaches to using rails to
    accomplish the query outlined above? i.e. I want to find all cars
    where at least 1 passenger carries a knife.

thanks!

As for “deduping” your result set, have a look here:
http://api.rubyonrails.org/classes/ActiveRecord/Base.html#M000992

You can pass the :select flag to find with something like this ->
“:select => ‘disctinct cars.*’”

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