Is there a better way to write this Rails model code?

[Michael Dippery]

I am working on an application that contains two models: Machine and MachineUpdate. A Machine has_many :machine_updates. The MachineUpdate class has a date field.

I want to find all Machines that do not have a MachineUpdate record for more than 2 weeks. Currently, I am using the following method on the Machine class to do so:

  def self.machines_to_check

    updates = MachineUpdate.find(:all, :conditions => "date < '#{UPDATE_THRESHOLD.days.ago}'")

    return nil unless updates.length > 0

    machines = []

    updates.each { |u| machines << u.machine }

    machines.uniq

  end

However, since a Machine is already connected to its updates via its machine_updates attribute, I wonder if there's a better way to do this? Also, the method doesn't work 100% correctly, as it won't find Machines that have had no updates (these machines should also be returned by the method).

Can anyone offer suggestions on how to clean this up?

Thanks,

-- Michael

[Nick Quaranto]

You could make a named scope for this...

class MachineUpdate

  named_scope :past, { :conditions => ["date < ?", UPDATE_THRESHOLD.days.ago] }

end

Then you should be able to do something like:  Machine.first.machine_updates.past

Definitely look more into named scopes if you haven't yet: http://api.rubyonrails.org/classes/ActiveRecord/NamedScope/ClassMethods.html

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[Nathan Fixler]

A named scope on the Machine class could definitely do it.

class Machine < ActiveRecord::Base

  has_many :machine_updates

  named_scope :with_updates, :include => :machine_updates, 

    :conditions => ["machine_updates.date < ?",UPDATE_THRESHOLD.days.ago]

end

Then just call Machine.with_updates.  Fewer SQL queries, and it returns an empty array instead of nil if there are no updates (as would be my preference).

[Marnen Laibow-Koser]

You're doing in the application something that should be done in the DB.  Also, some databases choke on "date" as a field name, and anyway, you should perhaps consider Rails' _at or _on magic...

Anyway, for your query...in SQL, what you want is this:

SELECT * FROM machines m LEFT JOIN machine_updates u ON (u.machine_id = m.id)

WHERE u.date > {cutoff date} AND u.machine_id IS NULL

(I know that looks self-contradictory, but that's really how left joins work...)

You could also use count(*) and HAVING, which might possibly be faster if you don't care about the individual dates. 

Turning these into AR finds is left as an exercise... :)

Can anyone offer suggestions on how to clean this up?

Thanks,

-- Michael

Best,

-- 

Marnen Laibow-Koser

http://www.marnen.org

mar...@marnen.org

[scott stewart]

+1 for the named_scope suggestion (Nathan)

On Dec 13, 5:08 pm, Marnen Laibow-Koser <mar...@marnen.org> wrote:

[Michael Dippery]

Thanks for the tips about named_scopes — didn’t know anything about them, and not only did they do the trick in this case, but I also managed to find a couple other uses for them in my app that have simplified the code a bit. Mucho gracias!


— Michael