select many through many... kind of - ruby-on-rails

Question

This is my first post on Stack, so please bear with me if I breach any protocol.
I'm working on a project in Rails (2.1.2), and I have a relations scenario that looks like this:
event [has_many] days
People (in different categories) can sign up for event days, giving the following binding results:
category [has_many] customers [has_many] orders [has_many] days
[belongs_to] event
Now, I'd like to have the total number of 'events' for one customer, or for all customers in a certain category, and I'm stuck. AFAIK, there's no way of performing a simple 'find' through an array of objects, correct? So, what would be the solution; nested loops, and a collect method to get the 'events' from the 'days' in orders?
Please let me know if I'm unclear.
Thanks for your help!

Answer 1

I would personally do this using a MySQL statement. I don't know for sure, but I think it is a lot faster then the other examples (using the rails provided association methods).
That means that in the Customer model you could do something like:
(Note that I'm assuming you are using the default association keys: 'model_name_id')
class Customer
def events
Event.find_by_sql("SELECT DISTINCT e.* FROM events e, days d, orders o, customers c WHERE c.id=o.customer_id AND o.id=d.order_id AND e.id=d.event_id")
end
end
That will return all the events associated with the user, and no duplicated (the 'DISTINCT' keyword makes sure of that). You will, as with the example above, lose information about what days exactly the user signed up for. If you need that information, please say so.
Also, I haven't included an example for your Category model, because I assumed you could adapt my example yourself. If not, just let me know.
EDIT:
I just read you just want to count the events. That can be done even faster (or at least, less memory intensive) using the count statement. To use that, just use the following function:
def event_count
Event.count_by_sql(SELECT DISTINCT COUNT(e.*) FROM ... ... ...
end

Answer 2

Your models probably look like this:
class Category
has_many :customers
class Customer
has_many :orders
has_many :days, :through => :orders # I added this
belongs_to :category
class Order
has_many :days
belongs_to :customer
class Day
belongs_to :event
belongs_to :order
class Event
has_many :days
With this you can count events for customer:
events = customer.days.count(:group => 'event_id')
It will return OrderedHash like this:
#<OrderedHash {1=>5, 2=>13, 3=>0}>
You can get events count by:
events[event_id]
events[1] # => 5
events[2] # => 13
etc.
If you want total number of uniq events:
events.size # => 3
In case of counting events for all customers in category I'd do something like this:
events = {}
category.customers.each {|c| events.merge!(c.days.count(:group => 'event_id') ) }
events.size # => 9 - it should be total number of events
But in this case you lose information how many times each event appeared.
I didn't test this, so there could be some mistakes.