如何从 peewee 的日期时间中减去时间增量？

Question

考虑下表：

class Recurring(db.Model):
    schedule    = ForeignKeyField(Schedule)
    occurred_at = DateTimeField(default=datetime.now)

class Schedule(db.Model):
    delay = IntegerField() # I would prefer if we had a TimeDeltaField

现在，我想要获取所有应该重复发生的事件：

query = Recurring.select(Recurring, Schedule).join(Schedule)
query = query.where(Recurring.occurred_at < now - Schedule.delay) # wishful

不幸的是，这不起作用。因此，我目前正在做如下事情：

for schedule in schedules:
    then  = now - timedelta(minutes=schedule.delay)
    query = Recurring.select(Recurring, Schedule).join(Schedule)
    query = query.where(Schedule == schedule, Recurring.occurred_at < then)

但是，现在我不是执行一个查询，而是执行多个查询。

有没有一种方法可以只使用一个查询来解决上述问题？我想到的一种解决方案是：

class Recurring(db.Model):
    schedule     = ForeignKeyField(Schedule)
    occurred_at  = DateTimeField(default=datetime.now)
    repeat_after = DateTimeField() # repeat_after = occurred_at + delay

query = Recurring.select(Recurring, Schedule).join(Schedule)
query = query.where(Recurring.repeat_after < now)

但是，上面的schema违反了第三范式的规则。

Answer 1

每个数据库都实现了不同的日期时间添加功能，这很糟糕。所以这将取决于您使用的是什么数据库。

例如，对于 postgres，我们可以使用 "interval" 助手：

# Calculate the timestamp of the next occurrence. This is done
# by taking the last occurrence and adding the number of seconds
# indicated by the schedule. 
one_second = SQL("INTERVAL '1 second'")
next_occurrence = Recurring.occurred_at + (one_second * Schedule.delay)

# Get all recurring rows where the current timestamp on the
# postgres server is greater than the calculated next occurrence.
query = (Recurring
         .select(Recurring, Schedule)
         .join(Schedule)
         .where(SQL('current_timestamp') >= next_occurrence))

for recur in query:
    print(recur.occurred_at, recur.schedule.delay)

如果您愿意，也可以用日期时间对象代替 "current_timestamp"：

my_dt = datetime.datetime(2019, 3, 1, 3, 3, 7)
...
.where(Value(my_dt) >= next_occurrence)

对于 SQLite，你会做：

# Convert to a timestamp, add the scheduled seconds, then convert back
# to a datetime string for comparison with the last occurrence.
next_ts = fn.strftime('%s', Recurring.occurred_at) + Schedule.delay
next_occurrence = fn.datetime(next_ts, 'unixepoch')

对于MySQL，你会做：

# from peewee import NodeList
nl = NodeList((SQL('INTERVAL'), Schedule.delay, SQL('SECOND')))
next_occurrence = fn.date_add(Recurring.occurred_at, nl)

最后，我建议您为 models/fields 尝试更好的名字。即，Schedule.interval 而不是 Schedule.delay，Recurring.last_run 而不是 occurred_at。

如何从 peewee 的日期时间中减去时间增量？

How to subtract a timedelta from a datetime in peewee?

python

postgresql

peewee