Question

I need to query multiple entities, something like session.query(Entity1, Entity2), only from a subquery rather than directly from the tables. The docs have something about selecting one entity from a subquery but I can’t find how to select more than one, either in the docs or by experimentation.

My use case is that I need to filter the tables underlying the mapped classes by a window function, which in PostgreSQL can only be done in a subquery or CTE.

EDIT: The subquery spans a JOIN of both tables so I can’t just do aliased(Entity1, subquery).

Answer 1

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class A(Base):
    __tablename__ = "a"

    id = Column(Integer, primary_key=True)
    bs = relationship("B")

class B(Base):
    __tablename__ = "b"

    id = Column(Integer, primary_key=True)

    a_id = Column(Integer, ForeignKey('a.id'))

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
s = Session(e)
s.add_all([A(bs=[B(), B()]), A(bs=[B()])])
s.commit()

# with_labels() here is to disambiguate A.id and B.id.
# without it, you'd see a warning
# "Column 'id' on table being replaced by another column with the same key."
subq = s.query(A, B).join(A.bs).with_labels().subquery()


# method 1 - select_from()
print s.query(A, B).select_from(subq).all()

# method 2 - alias them both.  "subq" renders
# once because FROM objects render based on object
# identity.
a_alias = aliased(A, subq)
b_alias = aliased(B, subq)
print s.query(a_alias, b_alias).all()