#1396 confirmed

SEL not carried through for associations

Reported by thomas | August 28th, 2010 @ 06:53 PM

Not sure if this is a bug or a new feature.

According to http://datamapper.org/articles/spotlight_on_laziness.html, SEL supposed to avoid the n+1 query problem. It references Zoo.find(:all, :include => [ :animals ]).each {}. find is no longer in the current 1.0.0 api, so one can only assume that DM is taking care of this under the hood.

I found a case (http://pastie.org/1119328) where each iteration of
Device.all(:limit => 100, :offset => k).each {|i| r << i.x} triggers a separate join request for each association. This loop took close to 3 seconds to perform. When the associations were removed [with all data defined in the same Device table] the same loop took only 68ms.

So, yes, n+1 is a performance killer in some cases. Per Dan Kubb's suggestion, I got around this by creating a view of the associations and define a model on top of this view. It would be nice to have DM handle this automatically.

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