Flask-sqlalchemy query datetime intervals

问题

I have defined a table with flask-sqlalchemy. Displayed below.

class Notes(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    notes = db.Column(db.Text, nullable=False)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
    added_at = db.Column(db.DateTime, default=db.func.now())

@staticmethod
def newest(num):
    return Notes.query.order_by(desc(Notes.added_at)).limit(num)

I'm attempting to write a query that is to replace and already existing direct query, which looks like this.

select notes,user,added_at from notes where added_at >= now() - INTERVAL 8 HOUR;

However based on the documentation that I can find, I'm not able to find a method to do the same. I'm able to make simpler queries, but I'm struggling to recreate what's pretty simple in sql itself.

I'm more than willing to read some documentation surrounding it, wasn't able to precisely nail that down either. Any direction you could provide would be awesome.

回答1:

I always have Python's datetime library get me the "now" and "8 hours ago", then just do a filter using the datetimes:

from datetime import datetime, timedelta

now = datetime.now()
eight_hours_ago = now - timedelta(hours=8)

Notes.query.filter(Notes.added_at > eight_hours_ago).filter(Notes.added_at < now).all()

回答2:

Can also try

from sqlalchemy import func, text

@staticmethod
def newest(num):
    return Notes.query.filter(Notes.added_at >= (func.date_sub(func.now(),  text('INTERVAL  8 HOUR')).order_by(desc(Notes.added_at)).limit(num)

from datetime import datetime, timedelta
from dateutil import tz 

@staticmethod
def newest(num):
    recent = datetime.now(tz=tz.tzlocal()) - timedelta(hours=8)

    return Notes.query.filter(Notes.added_at >= recent).order_by(desc(Notes.added_at)).limit(num)

回答3:

You can try something like

Notes.query.order_by(desc(Notes.added_at)).filter(
    Notes.added_at >= text('NOW() - INTERVAL 8 HOURS').limit(num)

As I only use pure sqlalchemy I tested this out with this syntax:

>>> from sqlalchemy import text
>>> # s is a standard sqlalchemy session created from elsewhere.
>>> print s.query(Notes).order_by(desc(Notes.added_at)).filter(
...     Notes.added_at >= text('NOW() - INTERVAL 8 HOURS'))
SELECT notes.id AS notes_id, notes.notes AS notes_notes, notes.added_at AS notes_added_at 
FROM notes 
WHERE notes.added_at >= NOW() - INTERVAL 8 HOURS ORDER BY notes.added_at DESC

Reason for using text for that section is simply because NOW() and INTERVAL usage is not consistent across all sql implementations (certain implementations require the use of DATEADD to do datetime arithmetic, and while sqlalchemy does support the Interval type it's not really well documented, and also on my brief testing with it it doesn't actually do what you needed (using example from this answer, for both sqlite and MySQL). If you intend to use the SQL backend as an ordered (but dumb) data store you can just construct the actual query from within Python, perhaps like so:

q = s.query(Notes).order_by(desc(Notes.added_at)).filter(
    Notes.added_at >= (datetime.utcnow() - timedelta(3600 * 8))
)

Some people dislike this as some databases (like postgresql) can deal with datetime better than Python (such as timedelta is ignorant of leap years, for instance).

回答4:

The following should also work:

from sqlalchemy import func
from sqlalchemy.dialects.postgresql import INTERVAL
from sqlalchemy.sql.functions import concat

Notes.query\
    .filter(
        Notes.added_at >= (func.now() - func.cast(concat(8, ' HOURS'), INTERVAL))
    )\
    .limit(num)

It has the nice property that 8 can be replaced with a value from inside the database, e.g., if you joined in another table with dynamic intervals. I gave this answer also here.

来源：https://stackoverflow.com/questions/30495935/flask-sqlalchemy-query-datetime-intervals

标签

python

flask-sqlalchemy