tom pisze:
> Hi group,
>
> i build a model to save measurement data. a measurement entry has
> always a channel (that is the source where the data comes from), a
> value (for example 1.04), a unit (for example m/s or Hz) and a
> timestamp.
> The timestamp and the channel are together unique. every channel can
> only have one data entry for one timestamp.
>
> i have the following model:
>
> class Data(models.Model):
> datetime = models.DateTimeField(help_text='the date and time of
> the measurement value', db_index=True)
> channel = models.CharField(help_text='the channelname',
> max_length=20, db_index=True)
> value = models.FloatField(help_text='the measurement value',
> db_index=True)
> unit = models.CharField(help_text='the unit of the measurement
> value', max_length=20, db_index=True)
> class Meta:
> unique_together = [('datetime', 'channel')]
>
> now, i want to have a query to get the following data-table:
>
> datetime channel1 unit1 channel2
> unit2
> 2009-01-01 00:00:00 10.0 m/s 1000 Hz
> 2009-01-01 00:00:10 15.0 m/s 1040 Hz
> 2009-01-01 00:00:20 16.0 m/s 1563 Hz
>
>
> Is something like this possible with django?
> i know that i can get all data ordered by date and the use python to
> get a datastructure like the described table. but i want to do it with
> the database because i think it's much faster.
>
> any ideas how to do this?
>
>
> Best regards,
>
> tom
>
> >
>
I would recommend a bit different solution. Instead of one, use two
tables like this:
class Entry(models.Model):
datetime = models.DateTimeField(help_text='the date and time of the
measurement value', db_index=True)
class Data(models.Model):
entry = ForeignKey(Entry)
channel = models.CharField(help_text='the channelname', max_length=20,
db_index=True)
value = models.FloatField(help_text='the measurement value', db_index=True)
unit = models.CharField(help_text='the unit of the measurement value',
max_length=20, db_index=True)
Then you have one entry element with as many data elements as you like. You
access it by taking one element object and going through relation, eg:
e = Entry(pk=1)
e.data_set.all()
I don't know if "unique_together" will work with this though
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