Hmm, to me 'last' logically operates on the return result of the query, which I would expect to be a collection. If you were to optimize this:
Task.all(:limit => 10, :order => [ :ordering.asc ]).last It seems like behind the scenes it should be turned into something like one of these: Task.all(:order => [ :ordering.asc ])[9] Task.first(:offset => 9, :order => [ :ordering.asc ]) Both of which optimize to: SELECT "id", "title", "ordering" FROM "tasks" ORDER BY "ordering" LIMIT 10 OFFSET 9 I'll probably go with the '.first' one since it's clear where the query ends, but just sayin'. I think it can be done without sub queries. I don't really think the user should run into DM Query objects unless they're looking for them. Thanks for a great tool! On Fri, Jan 14, 2011 at 2:36 PM, RipTheJacker <[email protected]> wrote: > I don't really think it's a bug. Calling Thing.all.last does a query > for the last item in the db, because Thing#all returns a DM Query, and > the #last updates that query. > > Here are ways using mysql: > http://www.mysqltutorial.org/select-nth-highest-record-database-table-using-mysql.aspx > > As you can see, they all rely on sub queries, some of which I don't > think can be done in DM without writing sql. > > Here are other approaches: > Thing.all(:limit =>10).to_a.last <- will find 10 records and return > the last of the array > Thing.all(:limit => 10) - Thing.all(:limit => 9) <- executes a single > query returning an array of the 1 item. > > I really think the offset approach you already figured out is the best > though. > > > On Jan 13, 9:45 pm, Robert Fletcher <[email protected]> wrote: > > Well, I discovered this workaround: > > > > a = Task.all(:order => [ :ordering.asc ])[9] > > > > Looks like this optimizes to what the previous one probably ought to. It > > uses OFFSET. > > > > > > > > On Thu, Jan 13, 2011 at 7:33 PM, lobati <[email protected]> wrote: > > > So I'm trying to get the nth result from the database: > > > > > a = Task.all(:limit => 10, :order => [ :ordering.asc ]).last > > > > > but this query, instead of returning the 10th item actually returns > > > the last item in the database. I get the same results if I do this: > > > > > a = Task.all(:limit => 10, :order => [ :ordering.asc ]) > > > a = a.last > > > > > However, I get the results I want if I do something like this: > > > > > a = Task.all(:limit => 10, :order => [ :ordering.asc ]) > > > a.each do |task| > > > # do nothing > > > end > > > a = a.last > > > > > I gather this is the result of some query optimization going on, but > > > this is a bug, right? So for the time being, is there are better way > > > of forcing the query to take place before '.last' and/or is there a > > > more efficient way for me to get the 10th record with the above > > > condition? > > > > > -- > > > You received this message because you are subscribed to the Google > Groups > > > "DataMapper" group. > > > To post to this group, send email to [email protected]. > > > To unsubscribe from this group, send email to > > > [email protected]<datamapper%[email protected]> > <datamapper%2Bunsubscribe@googlegrou ps.com> > > > . > > > For more options, visit this group at > > >http://groups.google.com/group/datamapper?hl=en. > > -- > You received this message because you are subscribed to the Google Groups > "DataMapper" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]<datamapper%[email protected]> > . > For more options, visit this group at > http://groups.google.com/group/datamapper?hl=en. > > -- You received this message because you are subscribed to the Google Groups "DataMapper" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/datamapper?hl=en.
