On Mon, Aug 6, 2018 at 8:01 PM, Nick Repole <nick.rep...@gmail.com> wrote: > Appreciate the response, that all makes sense to me. In hindsight, I'm > wondering how I ever expected there to be two Album objects with the same > identifier and different relationship collections... > > On a conceptual level, I wonder how feasible it'd be to have objects > expunged immediately as they're loaded. That way the parent Album with > album_id==1 could load, have it's tracks load, then when the child album is > loaded it wouldn't be the same Album object despite being album_id==1. > > My use case is purely read only, so I'm not particularly concerned with any > changes being tracked/persisted,so such a scenario would accomplish what I'd > need. But I realize my use case is pretty niche.
well it would be that the objects aren't put into the session at all as they are loaded, because you literally want the eager loading within the same load to create a new Album object. That wouldn't work because the loading mechanism is continuously seeing the same primary key at the top for that Album and pulling it from the identity map per-row in order to implement the eager loading in the first place. if you want to rely upon lazy loading, that is possible. The session supports lazy-loading on non-session-bound objects: http://docs.sqlalchemy.org/en/latest/orm/session_api.html#sqlalchemy.orm.session.Session.enable_relationship_loading . > > > > On Monday, August 6, 2018 at 9:58:33 AM UTC-5, Mike Bayer wrote: >> >> On Sat, Aug 4, 2018 at 11:27 PM, Nick Repole <nick....@gmail.com> wrote: >> > Hi, >> > >> > I'm attempting to load some filtered relationships, and am running into >> > issues when trying to independently load the same relationship multiple >> > times. >> > >> > As an example, I'd like to load Album.tracks, and >> > Album.tracks.album.tracks >> > in different ways. In the first tracks relationship, I'm trying to >> > include >> > all tracks with track_id >= 4, and in the second relationship every >> > trackId >> > < 4. >> > >> > Is this possible? And if so, what load options or technique should I be >> > using? The query generated below contains all the information I'm trying >> > to >> > get, it's a matter of loading that information into my models that's >> > tripping me up. >> > >> > Complete/runnable example can be found attached or below: >> >> great test case. What you're doing is not possible because you are >> looking at the "album.tracks" collection in two different contexts, >> and expecting them to have different contents, but this is the same >> Album object and the same "tracks" collection: >> >> (Pdb) results[0] >> <__main__.Album object at 0x7f2e1eea3898> >> (Pdb) results[0].tracks[0].album >> <__main__.Album object at 0x7f2e1eea3898> >> (Pdb) results[0].tracks is results[0].tracks[5].album.tracks >> True >> >> this is how the ORM identity map works, for any class / primary key, >> there's only one of them. >> >> now since this is SQLAlchemy, there are ways to do this, but it means >> you'd have two different Album objects with the same primary key in >> play. if you were to change both of them in a session and try to >> flush, you will get conflicts and/or errors. >> >> Here's the quickest way, note thre's now Album.tracks.parent_album for >> the real relationship: >> >> class Album(Base): >> >> __tablename__ = 'Album' >> >> album_id = Column("AlbumId", Integer, primary_key=True) >> title = Column("Title", Unicode(160), nullable=False) >> tracks = relationship("Track", backref="parent_album") >> >> >> class SubAlbum(Album): >> pass >> >> >> class Track(Base): >> >> __tablename__ = 'Track' >> >> track_id = Column("TrackId", Integer, primary_key=True) >> name = Column("Name", Unicode(200), nullable=False) >> album_id = Column("AlbumId", ForeignKey('Album.AlbumId'), index=True) >> album = relationship('SubAlbum') >> >> >> query = session.query(Album).filter(Album.album_id == 1) >> track_1_alias = aliased(Track, name="Track1") >> album_2_alias = aliased(SubAlbum, name="Album2") >> track_2_alias = aliased(Track, name="Track2") >> >> then your assertion passes. >> >> >> as you can see it is not possible for album<->track to be a >> bidirectional relationship and refer to two different Album objects at >> the same time. you have to choose one or the other. >> >> >> >> >> >> >> >> >> > >> > from sqlalchemy import create_engine >> > from sqlalchemy.orm import aliased, sessionmaker, relationship, >> > contains_eager >> > from sqlalchemy import Column, ForeignKey, Integer, Unicode >> > from sqlalchemy.ext.declarative import declarative_base >> > >> > Base = declarative_base() >> > metadata = Base.metadata >> > >> > >> > class Album(Base): >> > >> > __tablename__ = 'Album' >> > >> > album_id = Column("AlbumId", Integer, primary_key=True) >> > title = Column("Title", Unicode(160), nullable=False) >> > >> > >> > class Track(Base): >> > >> > __tablename__ = 'Track' >> > >> > track_id = Column("TrackId", Integer, primary_key=True) >> > name = Column("Name", Unicode(200), nullable=False) >> > album_id = Column("AlbumId", ForeignKey('Album.AlbumId'), >> > index=True) >> > >> > album = relationship('Album', backref="tracks") >> > >> > >> > engine = create_engine('sqlite://', echo=True) >> > Base.metadata.create_all(engine, checkfirst=True) >> > session_cls = sessionmaker(bind=engine) >> > session = session_cls() >> > >> > >> > # Set up the database with Chinook data >> > session.add(Album(album_id=1, title="For Those About To Rock We Salute >> > You")) >> > session.add(Album(album_id=2, title="Balls to the Wall")) >> > session.add(Album(album_id=3, title="Restless and Wild")) >> > session.add(Album(album_id=4, title="Let There Be Rock")) >> > session.add(Track(track_id=1, name="For Those About To Rock", >> > album_id=1)) >> > session.add(Track(track_id=2, name="Balls to the Wall", album_id=2)) >> > session.add(Track(track_id=3, name="Fast As a Shark", album_id=3)) >> > session.add(Track(track_id=4, name="Restless and Wild", album_id=3)) >> > session.add(Track(track_id=5, name="Princess of the Dawn", album_id=3)) >> > session.add(Track(track_id=6, name="Put The Finger On You", album_id=1)) >> > session.add(Track(track_id=7, name="Let's Get It Up", album_id=1)) >> > session.add(Track(track_id=8, name="Inject The Venom", album_id=1)) >> > session.add(Track(track_id=9, name="Snowballed", album_id=1)) >> > session.add(Track(track_id=10, name="Evil Walks", album_id=1)) >> > session.add(Track(track_id=11, name="C.O.D.", album_id=1)) >> > session.add(Track(track_id=12, name="Breaking The Rules", album_id=1)) >> > session.add(Track(track_id=13, name="Night Of The Long Knives", >> > album_id=1)) >> > session.add(Track(track_id=14, name="Spellbound", album_id=1)) >> > session.add(Track(track_id=15, name="Go Down" , album_id=4)) >> > session.add(Track(track_id=16, name="Dog Eat Dog", album_id=4)) >> > session.add(Track(track_id=17, name="Let There Be Rock", album_id=4)) >> > session.add(Track(track_id=18, name="Bad Boy Boogie", album_id=4)) >> > session.add(Track(track_id=19, name="Problem Child", album_id=4)) >> > session.add(Track(track_id=20, name="Overdose", album_id=4)) >> > session.add(Track(track_id=21, name="Hell Aint A Bad Place To Be", >> > album_id=4)) >> > session.add(Track(track_id=22, name="Whole Lotta Rosie", album_id=4)) >> > session.commit() >> > >> > >> > # Query set up >> > query = session.query(Album).filter(Album.album_id == 1) >> > track_1_alias = aliased(Track, name="Track1") >> > album_2_alias = aliased(Album, name="Album2") >> > track_2_alias = aliased(Track, name="Track2") >> > >> > # Subqueries we want to load our relationships from >> > # This will match 9 tracks for AlbumId==1 >> > track_1_subquery = session.query(track_1_alias).filter( >> > track_1_alias.track_id >= 4).subquery("Track1") >> > album_2_subquery = session.query(album_2_alias).subquery("Album2") >> > # This should only match 1 track for AlbumId==1 >> > track_2_subquery = session.query(track_2_alias).filter( >> > track_2_alias.track_id < 4).subquery("Track2") >> > >> > # Set up our joins >> > query = query.outerjoin(track_1_subquery, Album.tracks) >> > query = query.outerjoin(album_2_subquery, track_1_alias.album) >> > # Using album_2_alias.tracks for the second argument doesn't seem to >> > work >> > here. >> > query = query.outerjoin(track_2_subquery, >> > album_2_alias.album_id == >> > track_2_alias.album_id) >> > >> > >> > # Options - These don't work all at once, but seem fine separately >> > option1 = contains_eager("tracks", alias=track_1_subquery) >> > option2 = contains_eager("tracks.album", alias=album_2_subquery) >> > option3 = contains_eager("tracks.album.tracks", alias=track_2_subquery) >> > >> > >> > # Attempt to load just tracks >> > option3_query = query.options(option3) >> > results = option3_query.all() >> > assert(len(results) == 1) >> > # len here should be 9, but it's erroneously filtered to 1 >> > assert(len(results[0].tracks) == 1) >> > assert(len(results[0].tracks[0].album.tracks) == 1) >> > >> > -- >> > 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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