Thanks a ton for your help, Mike! We played around with it and are pretty happy with your solution using the load() event, so we'll be using that moving forward.
Damon On Wednesday, March 29, 2017 at 2:40:39 PM UTC-7, Mike Bayer wrote: > > I have a working version of both loading relationships via IN as well as > loading joined inheritance subclasses via IN, including your great idea > that extra eager loaders should continue to work for the subclass > loaders. > > I've only tested it with one scenario so far and both patches have a > long way to go re: tests and documentation. > > the scratch test case is currently in the commit message for the second > patch, which illustrates a base class + subclass load where both classes > have an additional relationship. This is at > https://gerrit.sqlalchemy.org/#/c/359/, note this builds upon the > previous gerrit at https://gerrit.sqlalchemy.org/#/c/352/. The work > at the moment will likely fall down with any kind of surprises but if > you wanted to start running it and finding those cases, that is always > helpful. > > So far this looks promising as something that can be in 1.2 with perhaps > some "EXPERIMENTAL" warnings surrounding it, but overall 1.2 was lacking > a "killer ORM feature" so these two would be it. > > > > On 03/23/2017 06:02 PM, mike bayer wrote: > > > > > > On 03/23/2017 02:40 PM, mike bayer wrote: > >> > >> > >> On 03/23/2017 12:53 PM, da...@benchling.com <javascript:> wrote: > >>> Hey Mike, > >>> > >>> Thanks for the quick response! > >>> > >>> For developers that are pretty familiar with the SQLAlchemy API, but > not > >>> so much the internals, would implementing the subqueryloads to > >>> contribute to SA be a reasonable endeavor? Could you ballpark how much > >>> time how long it might take for us to do it? > >> > >> I haven't looked at what this would take, but it would be intricate and > >> also need a lot of tests that are not easy to write. Like, if you > >> worked on it, you could probably get something working, but then that > >> probably wouldn't be how it really needs to be because all kinds of > >> things that are simple for simple cases don't work with the vast amount > >> of edge cases which we have. > > > > I've created an issue for both a new relationship loader and an > > inheritance loader at the same time, since they will use very similar > > paths, at https://bitbucket.org/zzzeek/sqlalchemy/issues/3944. A POC > > for the relationship loader so far looks to be very simple (but then > > again all the loaders start out very simple...) and is at > > https://gerrit.sqlalchemy.org/352. The same infrastructure and > > approach would also be used for the mapper inheritance loader, which > > would be enabled via a mapper()-level flag, as well as per-query using a > > new query option. > > > > I can't yet guarantee this will be a 1.2 thing, I'd have to get some > > more momentum going w/ test cases and all that. In a 1.2 release it > > would also be a little bit experimental as new loading styles usually > > have small issues coming up for months or years, as people try more use > > cases. > > > > > > > > > > > > > >> > >> The routine that's loading the additional columns just for one object > at > >> a time is here: > >> > >> > https://github.com/zzzeek/sqlalchemy/blob/master/lib/sqlalchemy/orm/loading.py#L635 > > >> > >> and then here for the bulk of it: > >> > https://github.com/zzzeek/sqlalchemy/blob/master/lib/sqlalchemy/orm/mapper.py#L2588 > > >> > >> > >> > >> But the much harder part would be how to work this step into the > loading > >> infrastructure, which would be somewhere in > >> > https://github.com/zzzeek/sqlalchemy/blob/master/lib/sqlalchemy/orm/loading.py#L273, > > > >> > >> which is a very intricate function with over a decade of constant > >> refactorings behind it, and I'd have to think pretty deeply for awhile > >> how best to do this. > >> > >> Not to mention that there's more than one way to do this query, there's > >> either re-using the criteria from the original query, or there's > >> injecting the primary key ids of the whole list of objects into an IN > >> clause after the fact. The latter approach is probably more efficient > >> but wouldn't work for composite primary keys outside of Postgresql. > As > >> a built in feature I'd want "IN" loading to be an option at least. > >> > >> > >> > >>> > >>> Regarding STI and relationships, is there any way to do that but still > >>> get the benefits of JTI? e.g. is there an easy way to resolve > >>> my_base_class_inst.subclass_prop as a proxy to the subclass? We could > >>> roll our own using __getitem__ but it seems a little hairy. > >> > >> So proxying to a related item wouldn't be terrible, sure you could use > >> __getitem__ or you could also add descriptors to the primary class, > >> adding the descriptors to match the "info" could be automated as well > >> (or even do it in the other direction, add special descriptor to main > >> class == a column gets added to related class). > >> > >> > >> Doing the thing where you query() for all the related classes after the > >> fact yourself might not be that terrible. you can use the load() > event > >> which receives the query context that has the query you need already. > I > >> guess it's time for proof of concept. Here's that. You can see at > >> the end we load all the "bs" without any per-object load. > >> > >> from sqlalchemy import * > >> from sqlalchemy.orm import * > >> from sqlalchemy.ext.declarative import declarative_base > >> from sqlalchemy import event > >> > >> Base = declarative_base() > >> > >> > >> class A(Base): > >> __tablename__ = 'a' > >> id = Column(Integer, primary_key=True) > >> a1 = Column(String) > >> type = Column(String) > >> > >> __mapper_args__ = {'polymorphic_on': type} > >> > >> > >> class B1(A): > >> __tablename__ = 'b1' > >> id = Column(Integer, ForeignKey('a.id'), primary_key=True) > >> b1 = Column(String) > >> b_data = Column(String) > >> __mapper_args__ = {'polymorphic_identity': 'b1'} > >> > >> > >> class B2(A): > >> __tablename__ = 'b2' > >> id = Column(Integer, ForeignKey('a.id'), primary_key=True) > >> b2 = Column(String) > >> b_data = Column(String) > >> cs = relationship("C", lazy='subquery') > >> > >> __mapper_args__ = {'polymorphic_identity': 'b2'} > >> > >> > >> class C(Base): > >> __tablename__ = 'c' > >> id = Column(Integer, primary_key=True) > >> b2_id = Column(ForeignKey('b2.id')) > >> > >> > >> class B3(A): > >> __tablename__ = 'b3' > >> id = Column(Integer, ForeignKey('a.id'), primary_key=True) > >> b3 = Column(String) > >> b_data = Column(String) > >> __mapper_args__ = {'polymorphic_identity': 'b3'} > >> > >> > >> def _loader_for_cls(target, context): > >> orig_query = context.query > >> > >> target_cls = type(target) > >> > >> # take the original query and chance the entity to the subclass > >> q = orig_query.with_entities(target_cls) > >> > >> # defer everything that's not PK / polymorphic_on from A. this > whole > >> # bit is just to avoid all those extra columns > >> to_defer = [] > >> mapper = inspect(target).mapper > >> inherited = mapper.inherits > >> while inherited is not None: > >> for attr in inherited.column_attrs: > >> if not attr.expression.primary_key and \ > >> attr.expression is not inherited.polymorphic_on: > >> to_defer.append(attr.key) > >> for attr in inherited.relationships: > >> to_defer.append(attr.key) > >> inherited = inherited.inherits > >> q = q.options(*[defer(k) for k in to_defer]) > >> > >> # store this strong reference so recs don't get lost while > >> # iterating > >> return q.all() > >> > >> > >> @event.listens_for(A, "load", propagate=True) > >> def load_extra(target, context): > >> key = ('loader_by_cls', type(target)) > >> > >> if key not in context.attributes: > >> context.attributes[key] = _loader_for_cls(target, context) > >> > >> > >> e = create_engine("sqlite://", echo=True) > >> Base.metadata.create_all(e) > >> > >> > >> s = Session(e) > >> s.add_all([ > >> B1(b_data='b11', b1='b1', a1='b11'), > >> B2(b_data='b21', b2='b2', a1='b21', cs=[C(), C(), C()]), > >> B3(b_data='b31', b3='b3', a1='b31'), > >> B1(b_data='b12', b1='b1', a1='b12'), > >> B1(b_data='b13', b1='b1', a1='b13'), > >> B2(b_data='b22', b2='b2', a1='b22', cs=[C(), C()]), > >> B3(b_data='b32', b3='b3', a1='b12'), > >> B3(b_data='b33', b3='b3', a1='b33') > >> ]) > >> s.commit() > >> > >> > >> q = s.query(A).filter(A.a1.like('%2%')) > >> > >> result = q.all() > >> > >> print "----- no more SQL ----" > >> for b in result: > >> if isinstance(b, B1): > >> print b.b1 > >> elif isinstance(b, B2): > >> print b.cs > >> print b.b2 > >> elif isinstance(b, B3): > >> print b.b3 > >> > >> > >> > >> > >> > >>> > >>> > >>> Thanks again, > >>> Damon > >>> > >>> On Wednesday, March 22, 2017 at 3:59:45 PM UTC-7, Mike Bayer wrote: > >>> > >>> > >>> > >>> On 03/22/2017 02:17 PM, da...@benchling.com <javascript:> wrote: > >>> > Hey all, > >>> > > >>> > We were wondering if you had any advice on having a large (~10) > >>> number > >>> > of polymorphic subclasses for a single base class. Using > >>> > with_polymorphic: '*' causes SQLAlchemy to joinedload all > >>> subclasses > >>> > like this: > >>> > > >>> > SELECT ... > >>> > FROM base_table > >>> > LEFT OUTER JOIN sub_table_1 ON base_table.id > >>> <http://base_table.id> = sub_table_1.id <http://sub_table_1.id> > >>> > LEFT OUTER JOIN sub_table_2 ON base_table.id > >>> <http://base_table.id> = sub_table_2.id <http://sub_table_2.id> > >>> > ... > >>> > > >>> > Postgres buckles under too many joins, and these queries start > >>> taking a > >>> > really long time. > >>> > > >>> > One other note is that for most of our queries, only a few of > >>> these > >>> > sub-tables are actually needed, so most of the joins are wasted. > >>> > Unfortunately, ahead of time, we don't know which tables will be > >>> needed > >>> > -- we're relying on the discriminator. > >>> > > >>> > Ideally, we'd be able to specify that the ORM should > >>> subqueryload the > >>> > subclasses (and only execute subqueries on the types that are > >>> present). > >>> > This would have to happen both when querying the base table, but > >>> also > >>> > when accessing relationships. We'd want it to execute a query on > >>> the > >>> > base table, then execute one query for each present subclass. > >>> > > >>> > Another solution might be to use some kind of hook that > >>> > > >>> > - is executed after a query returns with results (or after a > >>> list of > >>> > models are added to the session?) > >>> > - groups the models by type and runs its own subqueries to load > >>> the data > >>> > > >>> > Any help here is greatly appreciated! > >>> > >>> > >>> The purpose of with_polymorphic is more about being able to > >>> filter on > >>> multiple classes at the same time, which is why it uses joins, but > >>> these > >>> don't scale to many subclasses. Adding a subquery load for the > >>> related tables would be something that the ORM can someday have as > a > >>> feature, but it would need a lot of tests to ensure it's working > as > >>> advertised. > >>> > >>> There's a lot of ways to get those other tables loaded but none of > >>> them > >>> look that great. Turning off with_polymorphic(), one approach > >>> is to > >>> collect all the distinct types and identifiers from your query > >>> result; > >>> then do a separate query for each subtype: > >>> > >>> result = session.query(BaseClass).filter(...).all() > >>> > >>> types = sorted([(type(obj), obj.id <http://obj.id>) for > obj > >>> in result], > >>> key=lambda t: t[0]) > >>> > >>> for type, ids in itertools.groupby(types, key=lambda t: > >>> t[0]): > >>> session.query(type).filter(type.id.in_(ids)).all() > >>> > >>> That will emit a query with an INNER JOIN for each class and will > >>> populate the remaining records in the identity map. The columns > >>> that > >>> are already loaded are not re-accessed, though the DBAPI will > still > >>> send > >>> them over the network to the cursor. You can try limiting the > >>> columns > >>> you query for in each statement as well by using the defer() > option. > >>> > >>> Another way is to use with_polymorphic() but to provide a > different > >>> kind > >>> of SQL statement, like a polymorphic_union(). This would be a > >>> UNION of > >>> statements that each have an inner join. the resulting SQL is a > >>> beast > >>> but it at least isn't using those left outer joins. I think you > >>> can > >>> probably use sqlalchemy.orm.util.polymorphic_union() directly to > get > >>> this UNION statement built up automatically. > >>> > >>> Still another way is to reorganize the mappings to use > single-table > >>> inheritance and relationship() to link out to the related table, > >>> then > >>> the normal "subqueryload" feature to load them as relationships. > >>> Even > >>> though this way is ugly, I might use this (short of implementing > the > >>> related table subqueryload feature) just to make things simple. > >>> > >>> > >>> Definitely a feature that should be added but that's not an > >>> immediate > >>> solution. > >>> > >>> > > >>> > > >>> > Thanks, > >>> > Damon > >>> > > >>> > -- > >>> > SQLAlchemy - > >>> > The Python SQL Toolkit and Object Relational Mapper > >>> > > >>> > http://www.sqlalchemy.org/ > >>> > > >>> > To post example code, please provide an MCVE: Minimal, Complete, > >>> and > >>> > Verifiable Example. 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