id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
306	cached date time values have greater precision then ones retrieved from db causing issues	scanner@…	Adrian Holovaty	"A problem with dates in objects in memory vs ones being instantiated
out of the database.

I have a data model for a ""music root"" that is a directory and two
datetime stamps, like so:

{{{
class MusicRoot(meta.Model):
    fields = (
        meta.CharField('directory', maxlength = 1024),
        meta.DateTimeField('last_scan_started', blank = True, null = True),
        meta.DateTimeField('last_scan_finished', blank = True, null = True),
        )
}}}

I also have a track model that has some fields and a ""last_scanner""
date time field, like so:

{{{
class Track(meta.Model):
    fields = (
        meta.CharField('title', maxlength = 512),
	... ...
        meta.DateTimeField('last_scanned', blank = True, null = True),
        )
}}}

Now, in order to figure out if a ""track"" file has been removed (and
thus the object should be deleted) I set ""last_scan_started"" on the
music root to be datetime.datetime.now().

Then I walk the directory finding all files that are ""tracks.""
When I find a track I always update the track.last_scanned field with
datetime.datetime.now().

After I am done walking all the files I do this simple operation to
find out which tracks are no longer in this music root:
{{{
        missing_tracks = tracks.get_list(last_scanned__lt = music_root.last_scan_started)
}}}

Now, earlier in the same function I had done:

{{{
        music_root.last_scan_started = datetime.datetime.now()
        music_root.save()
}}}

The problem? ""missing_tracks"" is _all the tracks_ in the music root.
Pounded my head for a couple of minutes and then realized this:

{{{
music_root.last_scan_started -> 2005-08-10 23:38:49.031080
}}}

Almost all the tracks, that were filled in via fetches from the db if
they were not cached (because where I had created the 'track' object
was in a sub-scope) were:

{{{
track.last_scanned -> 2005-08-10 23:38:49
}}}

so: 

{{{
2005-08-10 23:38:49 < 2005-08-10 23:38:49.031080
}}}

I worked around this by doing a: 

{{{
music_root = musicroots.get_object(pk = music_root.id)
}}}

which seems to work although it might not if any funky caching is
happening.

But I should not need to do that."	defect	closed	Database layer (models, ORM)	1.0	normal	fixed	datetime precision		Unreviewed	0	0	0	0	0	0