So I'd like to set up the below associations. An Event is a normal AR model. Stat is a normal AR model. I want a special stat on Event that is a DB computation (mostly just sums of each column) and a summary of all the Stats for that Event. The Summary isn't persisted and is marked as readonly. Unfortunately, preloading is important in this case (and thus why I went off on the group_by tangent, sorry if that wasn't clear).
I guess I could make a new model for Summary referencing the stats table which uses a special select and always adds the group by clause, however, that seemed like overkill. I really didn't want to run a loop over the returned stats since the DB can give me exactly what I'm looking for on the initial query. Ok, here's a slightly abridged version of what I thought might work: Event: has_many :stats has_one :summary_stat, :select => "sum(people) as people, ...", :class_name => 'Stat', :foreign_key => 'event_id', :readonly => true Stat: belongs_to :event Adding "group" to the has_one solves the issue, but am I going about this in the "right" way? Thanks for the input! tony On Aug 30, 10:04 am, Matt Jones <[email protected]> wrote: > The "Summary" object you've described doesn't sound like a proper > ActiveRecord object - is it actually persisted to the database? It > sounds like you'd be better off just defining a non-AR model (maybe an > OpenStruct) for the summary. It's not going to help with preloading, > but you didn't state if that was an issue. > > --Matt Jones > > On Aug 28, 10:25 pm, Tony <[email protected]> wrote: > > > Hey all, > > > I've been playing around with special relationships (specifying the > > select, etc) on my models and trying to get Rails to return the > > results of database computations as part of an association. I've got > > one model representing an object and another model representing > > statistics about that object on per day basis. That main Event object > > has many Stats but it also has one Summary (of those Stats). > > Unfortunately, Rails won't allow me to have a has one association with > > computed columns (sums in my case) since it will not allow the use of > > the group by clause without which Rails can't connect the returned > > stats to the event since the event_id column is null. I've gotten > > around this by using a has_many association for the stats even though > > I'm grouping on the foreign key and will only ever have one record at > > most. This creates some really "hacky-looking" code since I have to > > read the first element of the returned array everywhere to read the > > summary stats. I went digging through the Rails source to take a look > > at why this might be prevented and I've come up with two thoughts: > > > 1) Rails should allow use of :group for a has_one association for > > specific use cases such as this one and let the programmer decide how > > it works. The fix is as simple as adding it to the > > valid_keys_for_has_one_association in associations.rb. However, this > > lead me to a second thought... > > > 2) In a has_one association it seems like it would never be > > detrimental to use a group by clause on the foreign key as this would > > always return one or fewer results for the parent table. This relates > > to the comment at association_preload.rb:148 (in 2.3.3) which states > > that there is no way to ask the database for one row per distinct > > foo_id (which is what group by would do, at least in MySQL). > > > Am I missing something or is this something that ought to be patched? > > Thanks for the input and help! > > > tony --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Ruby on Rails: Talk" 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/rubyonrails-talk?hl=en -~----------~----~----~----~------~----~------~--~---
