Complex rails find ordering - sql

Question

I am trying to do a find which orders results by their house name and then by the customer's last name.
Customer.find(:all,
:conditions =>['customers.id IN (?)', intersection],
:joins => 'JOIN histories ON histories.customer_id = customers.id
JOIN houses ON histories.house_id = houses.id',
:order => "houses.name ASC, customers.last_name ASC",
:select => "customers.*, histories.job_title, houses.name"
)
My problem is this will return every history related to each customer.
if I add AND histories.finish_date IS NULL
This will prevent every history for the selected customer being returned but it will also stop customers in the intersection who have no history or a finish_date set from being returned.
Basically I need every customer in the intersection returned once with there current house name(if they have one) and then ordered by their house name and then their last name.
So is there a way of doing this?
Here is an example
customer
id last_name
1 franks
2 doors
3 greens
histories
id finish_date house_id customer_id
1 NULL 1 1
2 NULL 2 2
3 11/03/10 2 1
4 22/04/09 1 2
NULL = current house
houses
id name
1 a
2 b
Results
intersection = 1,2,3
last_name house
franks a
doors b
greens NULL
Thanks

Answer 1

I think you need to use outer joins.
For example, this should work:
Customer.find(:all,
:conditions =>['customers.id IN (?) and histories.finish_date is null', intersection],
:joins => 'LEFT OUTER JOIN histories ON histories.customer_id = customers.id
LEFT OUTER JOIN houses ON histories.house_id = houses.id',
:order => "houses.name ASC, customers.last_name ASC",
:select => "customers.*, histories.job_title, houses.name"
)
If you've got an association between Customer and History and between History and House you should be able to do :include => [:histories => :house] instead of the :joins option.
The only other thing is that the customers with no house will appear first in the list due to NULL being earlier in the order than a non-NULL value. You might want to try an order option like this :
:order => 'isnull(houses.name), houses.name, customers.last_name'
to achieve what you specified.

Answer 2

IMO it's simpler to do the sorting logic in Rails instead of the database:
customers = Customer.find(:all, :conditions => { :id => intersection }, :include => [ { :histories => :houses } ])
customers.sort_by { |c| c.last_name }
customers.sort_by do |c|
current_house = c.histories.find_by_finish_date(nil) # Returns nil if no matching record found
if current_house
current_house.name
else
''
end
end
Explanations
:conditions can take an hash { :column_name => array } which translates into your IN where-condition
:include pre-loads (eager loading) the tables if the corresponding associations exist. To put it another way: :joins creates INNER JOINs, while :include creates LEFT JOINs. Here we will left join histories and again left join houses. You could omit this :include tag, in which case rails does a new query each time you access a histories or houses property.
sort_by allows to define a custom sort criteria.
find_by_finish_date is one of rails' magic methods; it is equivalent to h.find(:conditions => {:finish_date => nil })
How to output: Just output all of them in your view. If he does not have histories, customer.histories is an empty array.