Seems like exactly what I need! Many thanks for all your help, I will try that out right away :)
On Wednesday, April 1, 2015 at 7:04:51 PM UTC+2, Michael Bayer wrote: > > > > On 4/1/15 10:28 AM, Pierre B wrote: > > Here's a simple visual of the schema > > > OK, so that's called a polymorphic foreign key. SQLAlchemy doesn't have > first class support for this concept because it's relationally incorrect, > but there is an example at > http://docs.sqlalchemy.org/en/rel_0_9/_modules/examples/generic_associations/generic_fk.html > > which shows one way to produce this effect. The key aspects to this in > reference to your model attempts are that there are no ForeignKey objects; > objects like Column and ForeignKey are schema-level objects, and if you > construct one, that implies it exists in the schema. That's why you can't > make two Column objects with the same name pointing to the same table, and > this is what I'm referring to when I say that the schema has to be > considered when building out these declarations. > > The techniques to make the relationship here involve using the > "primaryjoin" argument to establish how the tables join directly, as well > as the "foreign()" annotation and/or "foreign_keys" argument which you can > see used in the example; that is, how the tables join is constructed using > all ORM constructs and not schema-level constructs. > > > > > <https://i.imgur.com/TaC2V6b.png> > > There are no foreign key constraints in the database schema, id1 and id2 > are just stored there, a type column is used to retrieve records e.g type > equals B for an association between RightB and ReftB and equals A between > RightA and LeftA. That is why I'm trying to set a default value for the > type column so I don't have to deal with that junction table when inserting > records. > > > On Wednesday, April 1, 2015 at 4:11:03 PM UTC+2, Michael Bayer wrote: > > > > On 4/1/15 4:55 AM, Pierre B wrote: > > Unfortunately I'm inheriting the relational model from an old application. > I have dozens of tables using a single junction table for associations. > I can not completely redesign my relational model because it needs to be > compatible with the old application. > > I was asking no such thing. I only ask that you consider the relational > model when building *new* elements of the application. If these models > are in fact mapping to an existing schema, I find it surprising that your > existing database schema includes *two* foreign key constraints present on > each of people4l2.id1 and people4l2.id2, constraining each column to both > left1.id/left2.id and right1.id/right2.id. > > > > > At this point, I think my best option is setting up table inheritance > at the database level (database is Postgresql) and migrating records into > children tables. Minimal code refactoring would be involved in the old > application and it would be possible to use the association object pattern. > > On Tuesday, March 31, 2015 at 8:05:19 PM UTC+2, Michael Bayer wrote: > > > > Pierre B <rocambol...@gmail.com> wrote: > > > I tried using the association object pattern before but can't get it to > work because I use the same id1 and id2 columns for all foreign keys and > I'm not able to override them in the sub-classes ("conflicts with existing > column" error). > > class MyClass(HasSomeAttribute, db.Model): > > __tablename__ = 'people4l2' > > id = db.Column(db.Integer, primary_key=True) > > > > class MySubClass1(MyClass): > > right1_id = db.Column('id2', db.Integer, ForeignKey('right1.id')) > > left1_id = db.Column('id1', db.Integer, ForeignKey('left1.id')) > > > > class MySubClass2(MyClass): > > right2_id = db.Column('id2', db.Integer, ForeignKey('right2.id')) > > left2_id = db.Column('id1', db.Integer, ForeignKey('left2.id’)) > > That’s because you do not have a __tablename__ for these subclasses, so > when > you put a column on the subclass, that is physically a column on the > ‘people4l2’ table; the names cannot be conflicting. Also, it is not > possible > to have a column named “people4l2.id2” which is in some cases a foreign > key > to “right1.id” and in other cases to “right2.id”. > > This probably all seems very complicated if you only think of it in terms > of > a Python object model. That’s why it is essential that you design your > database schema in terms of database tables, and how those tables will > work > within a purely relational model, without Python being involved, first. > > For simple cases, the design of the relational model and the object model > are so similar that this explicit step isn’t necessary, but once the goals > become a little bit divergent between relational and object model, that’s > when the relational model has to be developed separately, up front. This > is > the essence of how SQLAlchemy works, which becomes apparent the moment you > get into models like these which are typically impossible on most other > ORMs, since most ORMs do not consider design of the relational model as > separate from the object model. > > The tradeoff here is basically between “more work with SQLAlchemy” vs. > “not possible at all with other ORMs” :) > > The relational model is the more rigid part of the system here, so you > have to > work that part out first; then determine how you want to map the Python > object model on top of the relational model. > > > On Tuesday, March 31, 2015 at 4:29:52 PM UTC+2, Michael Bayer wrote: > > > > > > Pierre B <rocambol...@gmail.com> wrote: > > > > > Here's my use case: > > > right1 = Right() > > > right.left = Left() > > > > > > right2 = Right2() > > > right2.left = Left2() > > > > > > db.session.add(right) // automatically create the junction using > MySubClass1 and set the type field to 1 > > > db.session.add(right2) // automatically create the junction using > MySubClass1 and set the type field to 2 > > > db.session.commit() > > > > > > Basically I have a junction table associating a bunch of different > tables in my model. > > > I want to abstract that mechanism using relationships and polymorphism > so that I don't have to deal with that junction table while coding. > > > The relationships I created allow me to not have to deal with it while > selecting records but I can't get it to set the type field while inserting > records. > > > > OK, you are using the association object pattern. You cannot use > “secondary” > > in the way that you are doing here. You need to map a relationship to > > MySubClass1 explicitly. To reduce verbosity, you’d then apply the > > association proxy pattern. Without association proxy, your association > of > > right and left will be something like: > > > > right2 = Right2() > > right2.left_association = MySubClass1() > > right2.left_association.left = Left2() > > > > the association proxy then allows for MySubClass1() to be called > > automatically and you can refer to “right2.left” directly. > > > > Start with: > > > http://docs.sqlalchemy.org/en/rel_0_9/orm/basic_relationships.html#association-object > > > > > make that work completely, with the more verbose use pattern. > > > > then when that is totally working and understood, then move onto > association > > proxy: > > > > > http://docs.sqlalchemy.org/en/rel_0_9/orm/extensions/associationproxy.html > > > > > > > > > On Tuesday, March 31, 2015 at 4:11:51 PM UTC+2, Michael Bayer wrote: > > > > > > Pierre B <rocambol...@gmail.com> wrote: > > > > > > > I made a type in the Right model, here are the models again: > > > > > > if you’re referring to the behavior of Right.left when you use it in a > > > query, such as query(Right).join(Right.left), then the “default” value > of a > > > Column object has no interaction there. > > > > > > it seems like you probably want to do something very simple here but > I’m not > > > getting enough information on what that is. If you could illustrate > the > > > usage of the objects that you are looking for, that would help. > > > > > > > > > > > > > > > > > > > > class HasSomeAttribute(object): > > > > @declared_attr.cascading > > > > def type(cls): > > > > if has_inherited_table(cls): > > > > if cls.__name__ == 'MySubClass1': > > > > return db.Column(db.Integer, default=1) > > > > else: > > > > return db.Column(db.Integer, default=2) > > > > else: > > > > return db.Column(db.Integer, default=0) > > > > > > > > class MyClass(HasSomeAttribute, db.Model): > > > > __tablename__ = 'people4l2' > > > > id = db.Column(db.Integer, primary_key=True) > > > > id1 = db.Column(db.Integer) > > > > id2 = db.Column(db.Integer) > > > > > > > > class MySubClass1(MyClass): > > > > pass > > > > > > > > class MySubClass2(MyClass): > > > > pass > > > > > > > > class Right(db.Model): > > > > id = db.Column(db.Integer, primary_key=True) > > > > left = relationship( > > > > 'Left', > > > > secondary= MySubClass1.__table__, > > > > primaryjoin='and_(MySubClass1.type == 802, MySubClass1.id2 == > Right.id)', > > > > secondaryjoin='and_(MySubClass1.type == 802, MySubClass1.id1 > == Left.id)' > > > > ) > > > > > > > > class Left(db.Model): > > > > id = db.Column(db.Integer, primary_key=True) > > > > > > > > > > > > On Tuesday, March 31, 2015 at 12:12:35 PM UTC+2, Pierre B wrote: > > > > Hi Michael, > > > > > > > > Thank you for your response. > > > > Unfortunately I have already tried to use the __init__ > function/catch the init event but I am only referencing the sub classes in > a relationship which does not seem to actually instantiate classes because > the __init__ is never called/init event is never fired. > > > > Here is a simple version of my models. > > > > > > > > class HasSomeAttribute(object): > > > > @declared_attr.cascading > > > > def type(cls): > > > > if has_inherited_table(cls): > > > > if cls.__name__ == 'MySubClass1': > > > > return db.Column(db.Integer, default=1) > > > > else: > > > > return db.Column(db.Integer, default=2) > > > > else: > > > > return db.Column(db.Integer, default=0) > > > > > > > > class MyClass(HasSomeAttribute, db.Model): > > > > __tablename__ = 'people4l2' > > > > id = db.Column(db.Integer, primary_key=True) > > > > id1 = db.Column(db.Integer) > > > > id2 = db.Column(db.Integer) > > > > > > > > class MySubClass1(MyClass): > > > > pass > > > > > > > > class MySubClass2(MyClass): > > > > pass > > > > > > > > class Right(db.Model): > > > > id = db.Column(db.Integer, primary_key=True) > > > > subclass_attr = relationship( > > > > 'Contact', > > > > secondary= MySubClass1.__table__, > > > > primaryjoin='and_(MySubClass1.type == 802, MySubClass1.id2 > == Right.id)', > > > > secondaryjoin='and_(MySubClass1.type == 802, MySubClass1.id1 > == Left.id)' > > > > ) > > > > > > > > class Left(db.Model): > > > > id = db.Column(db.Integer, primary_key=True) > > > > > > > > MyClass is used as a junction table for a bunch of different > relationships, the type field is used to differentiate the relationships. > > > > > > > > > > > > On Monday, March 30, 2015 at 5:26:30 PM UTC+2, Michael Bayer wrote: > > > > > > > > > > > > Pierre B <rocambol...@gmail.com> wrote: > > > > > > > > > Hi all, > > > > > > > > > > I'm ultimately trying to have different default values for the > same column. Following the documentation, the @declared_attr.cacading > decorator seems to be the best approach. > > > > > Here's my code: > > > > > class HasSomeAttribute(object): > > > > > @declared_attr.cascading > > > > > def type(cls): > > > > > if has_inherited_table(cls): > > > > > if cls.__name__ == 'MySubClass1': > > > > > return db.Column(db.Integer, default=1) > > > > > else: > > > > > return db.Column(db.Integer, default=2) > > > > > else: > > > > > return db.Column(db.Integer, default=0) > > > > > > > > > > class MyClass(HasSomeAttribute, db.Model): > > > > > __tablename__ = 'people4l2' > > > > > id = db.Column(db.Integer, primary_key=True) > > > > > > > > > > class MySubClass1(MyClass): > > > > > pass > > > > > > > > > > class MySubClass2(MyClass): > > > > > pass > > > > > > > > > > I iterated quite a few times over this but I'm systematically > getting this error: > > > > > ArgumentError: Column 'type' on class <class > '__main__.MySubClass1'> conflicts with existing column 'people4l2.type’ > > > > > > > > this mapping illustrates MySubClass1 and MySubClass2 as both sharing > the > > > > same table “people4l2”, as they have no __tablename__ attribute, so > there > > > > can only be one “type” column. So in this case it is not appropriate > to use > > > > cascading in exactly this way, as MyClass already has a “type” > column, and > > > > that gets attached to the “people4l2” table and that’s it; there can > be no > > > > different “type” column on MySubClass1/MySubClass2. > > > > > > > > If you’d like “type” to do something different based on which class > is being > > > > instantiated, this is an ORM-level differentiation. Use either the > > > > constructor __init__() to set it or use the init() event > > > > ( > http://docs.sqlalchemy.org/en/rel_0_9/orm/events.html?highlight=event%20init#sqlalchemy.orm.events.InstanceEvents.init). > > > > > > > > > > OTOH if “type” is actually the “polymoprhic discriminator”, which is > what > > > > this looks like, then you’d be looking to just set up “type” as the > > > > “polymorphic_on” column and set up the “1”, “2”, “0” as the > polymorphic > > > > identity (see > > > > > http://docs.sqlalchemy.org/en/rel_0_9/orm/inheritance.html#single-table-inheritance > > > > > for a simple example). > > > > > > > > > > > > -- > > > > 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+...@googlegroups.com. > > > > To post to this group, send email to sqlal...@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+...@googlegroups.com. > > > To post to this group, send email to sqlal...@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 Go > > ... -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. 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