在专门的查询中设置group_by

Question

I need to perform data smoothing using averaging, with a non-standard group_by variable that is created on-the-fly. My model consists of two tables:

class WthrStn(models.Model):
  name=models.CharField(max_length=64, error_messages=MOD_ERR_MSGS)
  owner_email=models.EmailField('Contact email')
  location_city=models.CharField(max_length=32, blank=True)
  location_state=models.CharField(max_length=32, blank=True)
  ...

class WthrData(models.Model):
  stn=models.ForeignKey(WthrStn)
  date=models.DateField()
  time=models.TimeField()
  temptr_out=models.DecimalField(max_digits=5, decimal_places=2)
  temptr_in=models.DecimalField(max_digits=5, decimal_places=2)

  class Meta:
    ordering = ['-date','-time']
    unique_together = (("date", "time", "stn"),)

The data in WthrData table are entered from an xml file in variable time increments, currently 15 or 30 minutes, but that could vary and change over time. There are >20000 records in that table. I want to provide an option to display the data smoothed to variable time units, eg 30 minutes, 1, 2 or N hours (60, 120, 180, etc minutes)

I am using SQLIte3 as the DB engine. I tested the following sql, which proved quite adequate to perform the smoothing in 'bins' of N-minutes duration:

select id, date, time, 24*60*julianday(datetime(date || time))/N jsec, avg(temptr_out)
as temptr_out, avg(temptr_in) as temptr_in, avg(barom_mmhg) as barom_mmhg,
avg(wind_mph) as wind_mph, avg(wind_dir) as wind_dir, avg(humid_pct) as humid_pct,
avg(rain_in) as rain_in, avg(rain_rate) as rain_rate,
datetime(avg(julianday(datetime(date || time)))) as avg_date from wthr_wthrdata where
stn_id=19 group by round(jsec,0) order by stn_id,date,time;

Note I create an output variable 'jsec' using the SQLite3 function 'julianday', which returns number of days in the integer part and fraction of day in the decimal part. So, multiplying by 24*60 gives me number of minutes. Dividing by N-minute resolution gives me a nice 'group by' variable, compensating for varying time increments of the raw data.

How can I implement this in Django? I have tried the objects.raw(), but that returns a RawQuerySet, not a QuerySet to the view, so I get error messages from the html template:

  </p>
    Number of data entries: {{ valid_form|length }}
  </p>

I have tried using a standard Query, with code like this:

wthrdta=WthrData.objects.all()
wthrdta.extra(select={'jsec':'24*60*julianday(datetime(date || time))/{}'.format(n)})
wthrdta.extra(select = {'temptr_out':'avg(temptr_out)',
  'temptr_in':'avg(temptr_in)',
  'barom_mmhg':'avg(barom_mmhg)',
  'wind_mph':'avg(wind_mph)',
  'wind_dir':'avg(wind_dir)',
  'humid_pct':'avg(humid_pct)',
  'rain_in':'avg(rain_in)',
  'rain_sum_in':'sum(rain_in)',
  'rain_rate':'avg(rain_rate)',
  'avg_date':'datetime(avg(julianday(datetime(date || time))))'})

Note that here I use the sql-avg functions instead of using the django aggregate() or annotate(). This seems to generate correct sql code, but I cant seem to get the group_by set properly to my jsec data that is created at the top.

Any suggestions for how to approach this? All I really need is to have the QuerySet.raw() method return a QuerySet, or something that can be converted to a QuerySet instead of RawQuerySet. I can not find an easy way to do that.

Answer 1

The answer to this turns out to be really simple, using a hint I found from [ https://gist.github.com/carymrobbins/8477219][1]

though I modified his code slightly. To return a QuerySet from a RawQuerySet, all I did was add to my models.py file, right above the WthrData class definition:

class MyManager(models.Manager):
  def raw_as_qs(self, raw_query, params=()):
    """Execute a raw query and return a QuerySet. The first column in the
    result set must be the id field for the model.
    :type raw_query: str | unicode
    :type params: tuple[T] | dict[str | unicode, T]
    :rtype: django.db.models.query.QuerySet
    """
    cursor = connection.cursor()
    try:
      cursor.execute(raw_query, params)
      return self.filter(id__in=(x[0] for x in cursor))
    finally:
      cursor.close()

Then in my class definition for WthrData:

class WthrData(models.Model):
  objects=MyManager()
  ......

and later in the WthrData class:

  def get_smoothWthrData(stn_id,n):
    sqlcode='select id, date, time, 24*60*julianday(datetime(date || time))/%s jsec, avg(temptr_out) as temptr_out, avg(temptr_in) as temptr_in, avg(barom_mmhg) as barom_mmhg, avg(wind_mph) as wind_mph, avg(wind_dir) as wind_dir, avg(humid_pct) as humid_pct, avg(rain_in) as rain_in, avg(rain_rate) as rain_rate, datetime(avg(julianday(datetime(date || time)))) as avg_date from wthr_wthrdata where stn_id=%s group by round(jsec,0) order by stn_id,date,time;'
    return WthrData.objects.raw_as_qs(sqlcode,[n,stn_id]);

This allows me to grab results from the highly populated WthrData table smoothed over time increments, and the results come back as a QuerySet instead of RawQuerySet