SqlAlchemy filter by time difference

Question

I'm new (but not so new) to SQLAlchemy. I'm using version 0.9.3 for a project. I want to make a query to the SQLite database filtering the result to get those objects with no time stamp or have more than 24 hours since their last update (on the same column).

The problem is, I have no exact idea of how achieve the time filtering part, I did a IPython Notebook session, so people can look how long a tried looking for an answer to my issue:

In [1]:

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Unicode, DateTime, Integer, create_engine
from sqlalchemy.orm import sessionmaker, relationship, backref
from datetime import datetime

engine = create_engine('sqlite:///:memory:')
Base = declarative_base()
Session = sessionmaker(bind=engine)
session = Session()

In [2]:

class Product(Base):
    __tablename__ = "product"

    id = Column(Integer, primary_key=True)
    name = Column(Unicode(140), unique=True, nullable=False)
    department = Column(Unicode(20))
    added = Column(DateTime, default=datetime.now)
    last_time_parsed = Column(DateTime)

    def repr(self):
        return "<Product(name=%s, department=%s, added=%s, last_time_parsed=%s)>" % (
                            self.name, self.department, self.added, self.last_time_parsed)

In [3]:

# create the tables
Base.metadata.create_all(engine)

In [4]:

# create a false product
p1 = Product(name="Product X")
p2 = Product(name = "Product Y")
p3 = Product(name = "Product Z")

In [5]:

session.add(p1)
session.add(p2)
session.add(p3)
session.commit()

/home/jorge/projects/project1/lib/python2.7/site-packages/sqlalchemy/engine/default.py:525: SAWarning: Unicode type received non-unicode bind param value.
  param.append(processors[key](compiled_params[key]))

In [7]:

q = session.query(Product).filter(Product.last_time_parsed == None).filter(Product.last_time_parsed > 24)

In [9]:

print q.first().name

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-9-377361de0bab> in <module>()
----> 1 print q.first().name

AttributeError: 'NoneType' object has no attribute 'name'

In [14]:

q = session.query(Product).filter(Product.last_time_parsed == None)

In [15]:

print q.first().name

Product X

In [16]:

q = session.query(Product).filter(Product.last_time_parsed < 24)
print q.first().name

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-16-b10ccbb5d88d> in <module>()
      1 q = session.query(Product).filter(Product.last_time_parsed < 24)
----> 2 print q.first().name

AttributeError: 'NoneType' object has no attribute 'name'

In [20]:

q = session.query(Product).filter(Product.added > 24)
print q.first().name

Product X

In []:

In input 20 I know that Product.added is a DateTime data type (and I used just to test because is the only column holding DateTime values). What I don't understand is the logic statement. If were Product.added < 24 (as shown on input 16) instead of Product.added > 24 I would get no object from the database. So, by the number 24 what is actually the database understanding? 24 Minutes? 24 Hours? 24 Seconds? 24 Days?

So, again, How do I filter data to get data that have no time stamp or that was updated more than 24 hours ago?

Thanks :)

Answer 1

You do not need to use union, but simple OR part of the WHERE clause would do:

since = datetime.now() - timedelta(hours=24)
q = (session.query(Product).filter(or_(
            Product.last_time_parsed == None,
            Product.last_time_parsed < since)))