Django query hitting the db for every iteration

问题

I have a some model.py like so:

class Muestraonline(models.Model):
    accessionnumber = models.ForeignKey(Muestra, related_name='online_accessionnumber')
    muestraid = models.ForeignKey(Muestra)
    taxonid = models.ForeignKey(Taxon, null=True, blank=True)
    ...

class Muestra(models.Model):
    muestraid = models.IntegerField(primary_key=True, db_column='MuestraID') # Field name made lowercase.
    latitudedecimal = models.DecimalField(decimal_places=6, null=True, max_digits=20, db_column='LatitudeDecimal', blank=True) # Field name made lowercase.
    longitudedecimal = models.DecimalField(decimal_places=6, null=True, max_digits=20, db_column='LongitudeDecimal', blank=True) # Field name made lowercase.
    ...

And my view.py I want to get the unique specimen and find all specimens which share that taxonid. For the related specimens I just need the lat/long info:

def specimen_detail(request, accession_number=None, specimen_id=None):
    specimen = Muestraonline.objects.get(accessionnumber=accession_number, muestraid=specimen_id)
    related_specimens = Muestraonline.objects.filter(taxonid=specimen.taxonid).exclude(id=specimen.id)

    # create array for the related specimen points 
    related_coords = []
    # loop through results and populate array
    for relative in related_specimens:
        latlon = (format(relative.muestraid.latitudedecimal), format(relative.muestraid.longitudedecimal))
        related_coords.append(latlon) 
    related_coords = simplejson.dumps(related_coords)

But when I loop through related_specimens it ends up hitting the db once for each relative. Shouldn't I be able to get the latitudedecimal and longitudedecimal values in the format I need with only one extra db query? I know I missing something very basic in my approach here, just not sure of the best way to get this done.

Any help would be much appreciated.

回答1:

Just use select_related in your QuerySet:

related_specimens = Muestraonline.objects.filter(taxonid=specimen.taxonid).exclude(id=specimen.id).select_related('muestraid')

That will join your Muestraonline model to Muestra behind the scenes and each Muestraonline instance returned by the QuerySet will also contain a cached instance of Muestra.

来源：https://stackoverflow.com/questions/10255011/django-query-hitting-the-db-for-every-iteration