CODEWITHSUNDEEP
python-3.xsqlalchemyflask-sqlalchemy
Sergey Miletskiy
Sergey Miletskiy

Reputation: 487

Sqlalchemy filter by calculated datetime hybrid_property

I have a model Prescription.

from datetime import timedelta
from sqlalchemy.ext.hybrid import hybrid_property


class Prescription(db.Model):
    """ docstring """
    ID = db.column(db.Integer, primary_key=True)
    date = db.Column(db.DateTime)
    duration = db.Column(db.SmallInteger)

    @hybrid_property
    def expiration_date(self):
        # return self.date + timedelta(days=self.duration)
        return self.date + timedelta(days=7)

    @classmethod
    def actual(cls ):
        """ docstring """
        today = datetime.today()

        return cls.query.filter(Prescription.expiration_date>=today)

I want to get only actual prescriptions in my actual method, and when I specify

@hybrid_property
def expiration_date(self): 
    return self.date + timedelta(days=7)

everything works like a charm.

But every prescription has a different duration, and when I specify

@hybrid_property
def expiration_date(self): 
     return self.date + timedelta(days=self.duration)

I've got an error

TypeError: unsupported type for timedelta days component: InstrumentedAttribute

I tried to make a little hack like this 

@property
def days(self):
    return int(self.duration)

but no luck. Can anyone tell some workaround, or creating some lazy object for duration field or maybe another way get actual prescriptions, filtering by calculated expiration_date?

Upvotes: 2

Views: 1206

Answers (2)

Icebreaker454
Icebreaker454

Reputation: 1071

You might be trying to calculate a DATEDIFF: Calculate DATEDIFF in POSTGRES using SQLAlchemy

Here, is_expired will generate a SQL query part which calculates difference in days between the start date and utcnow(), and compares the result with self.duration

This works in PostgreSQL, but I have not tested on other RDBMS.

from datetime import datetime
import sqlalchemy as sa


class Prescription:
    ID = db.column(db.Integer, primary_key=True)
    date = db.Column(db.DateTime)
    duration = db.Column(db.SmallInteger)

    @hybrid_property
    def is_expired(self):
        days_since_published = sa.func.trunc((
            sa.extract('epoch', datetime.utcnow()) -
            sa.extract('epoch', self.date)
        ) / 3600 / 24)

        return days_since_published >= self.duration

    @classmethod
    def active(cls):
        return cls.query.filter(is_expired=False)

Upvotes: 3

Tech at The Sparks Foundation
Tech at The Sparks Foundation

Reputation: 1141

In your @classmethod annotated actual() method, you are trying to access a non-static property(expiration_date) which is raising the error. As per my understanding of what your code is doing, I have changed the code as follow:

from datetime import datetime,timedelta
from sqlalchemy.ext.hybrid import hybrid_property
from flask_sqlalchemy import SQLAlchemy
from flask import Flask

app = Flask(__name__)
db = SQLAlchemy(app)
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = True
app.config['SQLALCHEMY_DATABASE_URI'] ='sqlite:///'

class Prescription(db.Model):
    """ docstring """
    def mydefault(context):
        return context.get_current_parameters()['date'] + timedelta(days=context.get_current_parameters()['duration'])

    ID = db.Column(db.Integer,primary_key= True)
    date = db.Column(db.Date)
    duration = db.Column(db.SmallInteger)
    expiration_date = db.Column(db.Date,default = mydefault)

    @classmethod
    def actual(cls ):
        """ docstring """
        today = datetime.today().date()
        return cls.query.filter(Prescription.expiration_date>=today).all()
    def __repr__(self):
      return "Id:{} , date:{}, duration:{}, expiration_date:{}".format(self.ID,self.date,self.duration,self.expiration_date)

db.create_all()
q=Prescription(ID=1,date=datetime(2019,5,26).date(),duration = 1) #Expired
r=Prescription(ID=2,date=datetime(2019,5,20).date(),duration = 3) #Expired
s=Prescription(ID=3,date=datetime(2019,5,27).date(),duration = 5) #Not Expired
t = Prescription(ID=4,date=datetime.now().date(),duration = 1) #Not Expired

db.session.add(q)
db.session.add(r)
db.session.add(s)
db.session.add(t)
db.session.commit()

list_obj = Prescription.query.all()
print("All Objects in DB:-")
for l in list_obj:
  print(l)
print()
print("Valid Prescription:")
print(Prescription.actual())

Output:

Output

One thing I can suggest to you is since the model does not need the time of expiration. So you could change 

date = db.Column(db.DateTime)

to

date = db.Column(db.Date)

and it will ease your task :)

Upvotes: 0

Related Questions