On Aug 20, 2009, at 3:59 PM, Johannes Knopp wrote:
> > Hi *, > I am currently working on a Wikipedia API which means that we have a > database for each language we want to use. The structure of each > database is identical, they only differ in their language. The only > place where this information is stored is in the name of the database. > > When starting with one language the straight forward approach to use a > mapping between the tables to needed classes (e.g. Page) looked fine. > We defined an engine and corresponding metadata. When we added a > second > database with its own setup for engine and metadata we ran into the > following error: > > ------ > ArgumentError: Class '<class 'wp.orm.types.pages.Page'>' already has a > primary mapper defined. Use non_primary=True to create a non primary > Mapper. clear_mappers() will remove *all* current mappers from all > classes. > ------ > > I found an email saying that there must be at least one primary > mapper, > so using this option for all databases doesn't seem feasible. [1] if you'd like distinct classes to indicate that they "belong" in a different database, and you have very clear lines as to how this is performed, use the "entity_name" concept described at http://www.sqlalchemy.org/trac/wiki/UsageRecipes/EntityName . this sounds very much like your use case. > The next idea is to use sharding. For that we need a way to > distinguish > between the databases from the perspective of an instance, as noted in > the docs [2]: > "You need a function which can return a single shard id, given an > instance to be saved; this is called "shard_chooser" horizontal sharding is a method of storing many *homogeneous* instances across multiple databases, with the implication that you're creating one big "virtual" database among partitions - the main concept is that an individual instance gets placed in different partitions based on some ruleset. This is a little like your use case as well but since you have a very simple delineation i think the "entity name" approach is easier. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To post to this group, send email to sqlalchemy@googlegroups.com To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en -~----------~----~----~----~------~----~------~--~---
