Jack Christensen
2007-Feb-02 14:02 UTC
How to DRY has_many calculations working with many records
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class Person < ActiveRecord::Base
<br>
has_many :transactions
<br>
<br>
def balance
<br>
transactions.sum(:amount)
<br>
end
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end
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<br>
class Transaction < ActiveRecord::Base
<br>
belongs_to :person
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end
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<br>
Given the above, what is a good way to efficiently and DRYly find
people by balance?
<br>
<br>
Finding all records, then using the association is very slow.
<br>
Person.find(:all).select { <code
class="moz-txt-verticalline"><span
class="moz-txt-tag">|</span>p<span
class="moz-txt-tag">|</span></code>
p.balance > 100 }
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<br>
A subselect is much faster, but completely duplicates the logic.
<br>
Person.find(:all,
<br>
:select => "people.*, (SELECT SUM(amount) FROM
transactions WHERE
people.id=transactions.person_id) AS balance",
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:conditions => "(SELECT SUM(amount) FROM transactions
WHERE
people.id=transactions.person_id) > 100"
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<br>
Especially as the calculations get more complicated this starts to
become a real problem.
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<br>
Thanks.
<br>
<br>
<span class="moz-txt-tag">-- <br>
</span>Jack Christensen
<br>
<a class="moz-txt-link-abbreviated"
href="mailto:jackc-/SOt/BrQZzMOf2zXYvRtkodd74u8MsAO@public.gmane.org">jackc-/SOt/BrQZzMOf2zXYvRtkodd74u8MsAO@public.gmane.org</a>
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<pre class="moz-signature" cols="72">--
Jack Christensen
<a class="moz-txt-link-abbreviated"
href="mailto:jackc-/SOt/BrQZzMOf2zXYvRtkodd74u8MsAO@public.gmane.org">jackc-/SOt/BrQZzMOf2zXYvRtkodd74u8MsAO@public.gmane.org</a></pre>
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