Thank you very much MichaeI was in fact thinking that it was possible to not found support for that feature. But as you said, the compiler didn't shout at me and the application was running fine so I was thinking that I was doing things right. I'll go for the JOIN.
Best, Enrico On Thursday, January 9, 2014 8:15:14 PM UTC+1, Michael Bayer wrote: > > > On Jan 9, 2014, at 11:32 AM, Enrico Bottani <[email protected] <javascript:>> > wrote: > > Hello guys, > > I'm building a web service to provide data to our SDK and I'm facing a > wired problem. I have created 4 different models for my needs, a base class > that is called *Unit*, two derivatives from the base class *VideoUnit*, > *QuestionUnit*, and the last class is a derivative of both *VideoUnit*and > *QuestionUnit*, that is called, with lot of fantasy, *VideoQuestionUnit*. > Everything was working fine till the point I was trying to serialize this > data to be transmitted with JSON. > I successfully gather JSON from *VideoUnit* and *QuestionUnit *(thanks to > plaes<http://stackoverflow.com/questions/7102754/jsonify-a-sqlalchemy-result-set-in-flask>) > > but now I can't figure out why when I try to make the same thing with > *VideoQuestioUnit > *the program crash telling me: "*KeyError: 'questions'*" > I can imagine that there is something wrong when *VideoQuestionUnit* try > to access the "*questions"* data declared in *QuestionUnit*(also my dog > get that I know), but I don't understand what's wrong with that. > Did someone have an idea of what I'm doing wrong? > > > first of all, SQLAlchemy’s inheritance mapping system has no direct > support for multiple inheritance at all. True multi-table multiple > inheritance in SQL is a highly esoteric and complex pattern and I doubt > SQLAlchemy is ever going to go there, at least fully; just getting single > hierarchy inheritance to work fully has taken many years and was enormously > complicated. While I see here that at least VideoQuestionUnit doesn’t > have a table of its own, still the fact that it is subclassing two other > joined-inh subclasses isn’t something the ORM has any idea what to do with. > Loading a VideoQuestionUnit means it would need to join to both of the > inherited tables, like a polymorphic load does, but there’s no mechanism to > produce one object deriving from more than one mapped class. I’m surprised > the mapping doesn’t just fail immediately. > > secondly, well actually I’m not sure there’s a “secondly” here :). I > guess the way you can load and persist VideoQuestionUnit objects is by > mapping it to a JOIN of all three tables at once - and you’d also need to > probably add “concrete=True” to the mapping so that it doesn’t attempt to > use any of the superclass mappings for persistence or querying. Not > something I’ve tried though it might be workable. > > > > > > > > *CODE:* > class Unit(db.Model): > id = db.Column(db.Integer, unique = True, primary_key = True) > name = db.Column(db.String(128), index = True, unique = False) > type = db.Column(db.SmallInteger) > brand_id = db.Column(db.Integer, db.ForeignKey('brand.id')) > @property > def serialize(self): > print "Serialize on superclass" > pass > > class VideoUnit(Unit): > uri = db.Column(db.String(256), index = False, unique = False) > @property > def serialize(self): > return { > 'ID' : self.id, > 'Name' : self.name, > 'Link' : self.uri > } > > class QuestionUnit(Unit): > questions = db.relationship('Question', backref = 'QuestionUnit', lazy > = 'dynamic') > @property > def serialize(self): > return { > 'ID' : self.id, > 'Name' : self.name, > 'Questions' : self.serialize_questions > } > @property > def serialize_questions(self): > return [item.serialize for item in self.questions] > > class VideoQuestionUnit(VideoUnit, QuestionUnit): > @property > def serialize(self): > return { > 'ID' : self.id, > 'Name' : self.name, > 'Link' : self.uri, > 'Questions' : self.serialize_questions > } > @property > def serialize_questions(self): > return [item.serialize for item in self.questions] > > > > Thanks in advance, > > Enrico > > -- > 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 [email protected] <javascript:>. > To post to this group, send email to [email protected]<javascript:> > . > Visit this group at http://groups.google.com/group/sqlalchemy. > For more options, visit https://groups.google.com/groups/opt_out. > > > -- 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 [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/groups/opt_out.
