using Model.find for inner joins and to 'dedupe' a result set


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

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

  from cars c
    inner join passengers p on (p.car_id =
      inner join pocket_items pi on (pi.passenger_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.


As for "deduping" your result set, have a look here:

You can pass the :select flag to find with something like this ->
":select => 'disctinct cars.*'"