Newbie: Rails' way to query database in my case - ruby-on-rails

Question

I am using Ruby v1.8 and Rails v2.3.
I have a two model objects: Cars and Customers,
Model Cars:
class car < ActiveRecord::Base
#car has attribute :town_code
has_many :customers
end
Model Customers:
class customer < ActiveRecord::Base
# customer has attribute :first_name, :last_name
belongs_to :car
end
Now in my controller, I got a string from VIEW, and the received string has the format firstname.lastname#town_code, for example a string like "John.smith#ac01" which can be parsed as first_name="John", last_name="smith" and town_code="ac01"
Now I would like use the Rails's way to query the database to find all the customer objects (match the above conditions) from Customers table which has :
first_name="John",
last_name="smith"
and owned a car(by car_id) with car's town_code="ac01".
what is Rails' syntax to query this?
I know it should be something like (if I wanna count the nr of matched customer):
Customer.count :consitions =>{:first_name => "John", :last_name=>"smith"...}
But, I am not sure how to refer to a customer that has a referenced car with car's town_code= "ac01" ?
------------------ My question --------------------
I want to have two queries:
-one is used to count the number of matching customers,
-the other query returns the customers objects like find_by_ query.
What is the syntax in Ruby on Rails for the two queries?

Answer 1

It should be something similar to
Customer.where(:firstname => "John", :last_name => "Smith").count
If you have many Customers of Car, you can do something like
Car.where(...).customers.where(...)
You should really be firing rails c to test your queries in (I might be slightly off)

Answer 2

You could have something like:
#customers = car.where(:town_code => town_code).customers.where(:first_name => first_name, :last_name => last_name)
And then just count the results:
#customer_count = #customers.count
This assuming you parsed your string into the variables town_code, first_name, and last_name, like you said.
Edit
I don't think Rails v2.3 supports these chains of Active Record queries because I believe it lacks lazy loading from DB. I'm not completely sure. Also, I realize my first suggestion would't work because there could be many cars with the same town_code. I guess you could solve it using the map function like so (not tested):
#customers = car.all(:conditions => {:town_code => town_code}).map{ |c| c.customers.where(:first_name => first_name, :last_name => last_name) }
And then count them like before:
#customer_count = #customers.count

Answer 3

I believe you could do something like this: source
Customer.find(:all, :include => :car, :conditions => "customers.first_name = 'John' AND customers.last_name = 'Smith' AND cars.town_code = 'ac01'")
Counting all customers with a specification can be achieved by this command: source
Customer.count(:all, :include => :car, :conditions => "customers.first_name = 'John' AND customers.last_name = 'Smith' AND cars.town_code = 'ac01'")
By the way, if you are in the position to choose what you work with, I would advise you to go for Rails 3. The chaining methods described by Joseph would make this kind of query a lot easier and it'll save you upgrading issues down the road. (And you tagged the question for Rails 3)