Outstanding! My puny mind was just beginning to tickle this documentation and was starting to get the idea that this is the right direction. Good to know I'm on track.
Is it also possible that having a class which maps to a join* will help? Then rather than having PersonAction and PersonUnit I would just have Person. Would that simplify the relationships? I'll have a play around with this and see where I get. Thanks, - Peter * http://docs.sqlalchemy.org/en/rel_0_9/orm/mapper_config.html#maptojoin On 20 October 2014 18:26, Michael Bayer <mike...@zzzcomputing.com> wrote: > >> On Oct 20, 2014, at 1:14 PM, Peter Waller <pe...@scraperwiki.com> wrote: >> >> Hi All, >> >> I am wondering if it is possible to describe in sqlalchemy the >> following relationship with the new features introduced since 0.9.2. >> I've been reading this document and it looks close to what I'm trying >> to achieve: >> >>> http://docs.sqlalchemy.org/en/rel_0_9/orm/relationships.html#composite-secondary-joins >> >> As does this previous mailing list post: >> >>> https://groups.google.com/d/topic/sqlalchemy/ov-mYWA7XAM/discussion >> >> There is a pre-existing structure to deal with. I would rather not >> change the structure if it can be avoided, but may have to resort to >> doing so. Here are the tables: >> >> TableName (column, ...) >> >>> PersonAction (PersonID, ActionID) >>> PersonUnit (PersonID, UnitID) >>> DepartmentUnit (UnitID, DepartmentID) >>> ActionMeaning (DepartmentID, ActionID) > > So as far as the patterns at > http://docs.sqlalchemy.org/en/rel_0_9/orm/relationships.html, this one > qualifies for > http://docs.sqlalchemy.org/en/rel_0_9/orm/relationships.html#relationship-to-non-primary-mapper. > The reason is because PersonAction has a direct FK to ActionMeaning > (ActionID) as well as towards elements on what would be “secondary” here, > PersonUnit/DepartmentUnit. The criteria for that is: "we seek to join from A > to B, making use of any number of C, D, etc. in between, however there are > also join conditions between A and B directly.” > > So you map ActionMeaning using a non-primary mapper to a join of > ActionMeaning, DepartmentUnit, and PersonUnit, then PersonAction can relate > to this mapper directly. > > -- > You received this message because you are subscribed to the Google Groups > "sqlalchemy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sqlalchemy+unsubscr...@googlegroups.com. > To post to this group, send email to sqlalchemy@googlegroups.com. > Visit this group at http://groups.google.com/group/sqlalchemy. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/d/optout.
