bigdata——update

class models_a(models.Model):
  	value_a = models.FloatField(default=0.0)
  	value_a_1 = models.FloatField(default=0.0)
  	
class models_b(models.Model):
  	foreign = models.ForeignKey(models_a)
  	value_b = models.FloatField(default=0.0)
  	
####进行更新model_b里面的字段value_b,让他的值等于models_a里面的value_a和value_a_1和value_b这三个值之和
results = models_b.objects.exclude(value_b=0.0)
for i in results.iterator():
    i.value_b = i.value_b + i.foreign.value_a + i.foreign.value_a_1
    i.save()
###