Hi all, I've got a question that I hope generates healthy debate and perhaps even a solution for me. Without going into too much detail I'm working on a project in which we perform calculations on a large hierarchical data set. We haven't used the acts_as_tree or acts_as_nested plugins because each level in the hierarchy has a well defined role and various attributes that only fit at that level in the hierarchy. To give you a brief taste the hierarchy roughly goes: local government area, precinct, building type, consumption.
For a given local government area the number of records contained in the entire hierarchy is about 80000, the bulk of them being consumption records (since they are the leaves of the hierarchy). A feature of the project is that one should be able to perform a projection of consumptions into the future. This is a fairly complex algorithm and involves looking at all 80000 records and combining them in various ways. This algorithm, naively written, has a huge database latency. The bulk of the time is spent querying and receiving results from the database. We have had some success in optimising various parts of the algorithm by performing less queries. A lot of the time this means that we pull the records out of the database and put them into some kind of look-up structure (a hash with a key equal to the attributes (plural) of interest in the model). This allows us to do a kind of "in memory" query but, annoyingly, only on whatever we choose as the key for the hash. You can no longer perform general queries on the collection in memory. Basically we lose all the expressiveness/terseness of ActiveRecord. What we really want to be able to do is this: 1. We want to pull a large collection of objects from the database into memory 2. We want to be able to select subsets of these in-memory objects with a similar flexibility to querying them using ActiveRecord 3. After having updated them in memory we want to be able to write them back to the database. I should metion that none of their unique keys will have changed. We want this to be some kind of bulk update. Some approaches that I've thought of but I don't think work: - Simply caching will not work because, as far as I know, this only works when you perform the same query twice. We are not doing this. We're querying the collection in many different ways. - This isn't really an issue to do with the kind of database. Switching over to CouchDB or an Object Database doesn't obviously solve our problem. The problem is that although we know in advance that all our queries will returns results that are a subset of the collection of 80000 objects we want to be able to perform many different sorts of queries returning many different subsets of the 80000 objects. Having the objects all sitting in memory really seems to be the way to go. - We could also just create a class hierarchy that mirrors the hierarchy of the data and forget about ActiveRecord entirely. We could then just serialize this structure and write it to disk or a database. We don't get the advantage of being able to query the data structure this way though. Some final notes: a) This site will, at most, have a few simultaneous clients so having all 80000 records in memory should not be a problem. b) We've had some luck with bulk-update part of point 3 above. Zach Dennis' AR-extensions plug-in has been quite useful. This email is still not as clear as I wanted it to be even though I've spent some time on it. If you need any clarification please feel free to ask. Cheers, Sean -- You received this message because you are subscribed to the Google Groups "Ruby or Rails Oceania" 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/rails-oceania?hl=en.
