I would expect a Collection of the Model from which the query was executed, with any additional fields discarded.
What about all users who have never posted? I know this works right on the surface now, but it's hugely inefficient to the point it could never be used in production. SQL: SELECT users.* FROM users LEFT JOIN posts ON posts.user_id = users.id WHERE posts.id IS NULL (DataMapper builds a gigantic IN clause, which obviously you'd have to optimise out in production where you're looking at millions of rows) There are certainly other valid cases for left join being used as part of a more complex constraint. My issue is that the ORM wants to believe it should know better than the person trying to extract the data, which is basically a broken abstraction. I love DM, but it does run us into a corner quite often so we're holding out for v2 and hopefully we'll have a clearer idea of what the future is looking like for queries where you actually need to manipulate the join behaviour yourself. On 08/12/2011, at 8:40 AM, Jordan Ritter wrote: > > What type of concrete objects would you expect the ORM layer to return, if > such a method call existed? Outer joins tend to be used for getting mixed > tuples of data that don't correspond to concrete ORM types. I can't > immediately fathom what any ORM's behaviour should be.. > > What do you mean by "without talking to the data-store directly"? The only > way I see to do this is to pass the DM adapter the SQL directly, and process > the responses as abstract tuples of data (you'll get back an array). You > could hydrate objects yourself manually from there, but YMMV: with any kind > of outer join you can end up with nulls, and extra > check-fields-before-hydration logic would be crappy to deal with (and why > bother - this is DM's job to begin with). > > FWIW, you might consider using direct SQL to conduct your outer join(s) and > only retrieve primary keys, then use the ORM to hydrate the objects akin to > User.all(:id => [..]). > > cheers, > --jordan > > > On Dec 7, 2011, at 1:07 PM, terminal_breaker wrote: > >> Is there a way to do left outer joins in Datamapper without having to >> resort to talking to the data-store directly. I've been searching the >> net and the Datamapper docu with not success on details how to do so. >> >> For example: >> >> SELECT p.user_id, p.description, u.user_name >> FROM privileges p >> LEFT OUTER JOIN users u ON (p.user_id = u.id) >> WHERE p.description = "delete rights" >> >> And let's say that I have a User class and a Privilege class too. >> >> >> -- >> 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. >> > > > -- > 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. -- 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.
