Querying for overlapping time ranges in SQLAlchemy and Postgres - postgresql

Question

I'm using Flask-SQLAlchemy to describe a Postgres database. Three related tables look like this (in part):
from sqlalchemy.dialects.postgresql import TSTZRANGE
class Shift(Base):
__tablename__ = "shifts"
id = db.Column(db.Integer, primary_key=True)
hours = db.Column(TSTZRANGE, nullable=False)
class Volunteer(Base):
__tablename__ = "volunteers"
id = db.Column(db.Integer(), primary_key=True)
shifts = db.relationship(
"Shift",
secondary="shift_assignments",
backref=db.backref("volunteers", lazy="dynamic"),
)
class ShiftAssignment(Base):
__tablename__ = "shift_assignments"
__table_args__ = (db.UniqueConstraint('shift_id', 'volunteer_id', name='_shift_vol_uc'),)
id = db.Column(db.Integer, primary_key=True)
shift_id = db.Column("shift_id", db.Integer(), db.ForeignKey("shifts.id"))
volunteer_id = db.Column(
"volunteer_id", db.Integer(), db.ForeignKey("volunteers.id")
)
Now, I'm assigning a Volunteer to new Shift and want to make sure that the vol isn't already committed to a different Shift at the same time.
I have tried this in a Volunteer instance method, but it's not working:
new_shift = db.session.get(Shift, new_shift_id)
if new_shift not in self.shifts:
for shift in self.shifts:
overlap = db.session.scalar(shift.hours.overlaps(new_shift.hours))
This results in the following exception:
'DateTimeTZRange' object has no attribute 'overlaps'
It seems like I should probably not even be doing this by iterating over the list anyway, but should be directly querying the DB to do the date overlap math. So I guess I need to join the volunteers and shifts and then filter to find out if any shifts overlap with the target shift. But I can't figure out how to do that, and examples of overlaps and its RangeOperators friends is really thin on the ground.
Would appreciate a hand here.

Answer 1

It was much easier than I was making it. Again, this is in a Volunteer instance method.
new_shift = db.session.get(Shift, new_shift_id)
overlapping_shift = (
db.session.query(Shift, ShiftAssignment)
.join(ShiftAssignment)
.filter(ShiftAssignment.volunteer_id == self.id)
.filter(Shift.hours.overlaps(new_shift.hours))
.first()
)
if overlapping_shift:
print("overlap found")
Note that the query returns a (Shift, ShiftAssignment) tuple. We join the two appropriate tables and then filter twice, left with any overlapping shifts assigned to the current volunteer.