SQLAlchemy order result by function
This is the code I have and it works (returns all problems, ordered by difficulty):
def get_noteworthy_problems(self):
ACategory = aliased(Category)
AProblem = aliased(Problem)
all_prob = DBSession.query(AProblem).filter(
AProblem.parent_id == ACategory.id,
ACategory.parent_id == self.id)
noteworthy_problems = \
sorted(all_prob, key=lambda x: x.difficulty(), reverse=True)
return noteworthy_problems
But I think I should optimize this code. Is it possible to change the code with order_by
and my function difficulty()
? My function returns a number. I tried something like:
result = DBSession.query(AProblem).filter(
AProblem.parent_id == ACategory.id,
ACategory.parent_id == self.id).order_by(
AProblem.difficulty().desc())
but I am getting an error TypeError: 'NoneType' object is not callable
.
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Hybrid attributes are special methods that act on both a Python property and an SQL expression. As long as your function difficulty
can be expressed in SQL, it can be used for filtering and ordering just like a regular column.
For example, if you are calculating difficulties because the number of parrots has a problem, ten times, if the problem is older than 30 days, you should use:
from datetime import datetime, timedelta
from sqlalchemy import Column, Integer, DateTime, case
from sqlalchemy.ext.hybrid import hybrid_property
class Problem(Base):
parrots = Column(Integer, nullable=False, default=1)
created = Column(DateTime, nullable=False, default=datetime.utcnow)
@hybrid_property
def difficulty(self):
# this getter is used when accessing the property of an instance
if self.created <= (datetime.utcnow() - timedelta(30)):
return self.parrots * 10
return self.parrots
@difficulty.expression
def difficulty(cls):
# this expression is used when querying the model
return case(
[(cls.created <= (datetime.utcnow() - timedelta(30)), cls.parrots * 10)],
else_=cls.parrots
)
and request it with:
session.query(Problem).order_by(Problem.difficulty.desc())
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