Multiple annotate with sum and display data in admin - Django

Question

I'm new to both Django and Python. Currently I'm trying the Django Admin by doing.

I've three models for a Django app, which are GoodsItem, SoldGoodsItem and FinishedGoodsItem. The models.py is:

from django.db import models


class GoodsItem(models.Model):
    name = models.CharField(max_length=255)
    size = models.DecimalField(max_digits=4, decimal_places=2)
    INCHES = 'IN'
    NUMBER = 'NUM'
    GOODS_ITEM_SIZE_UNITS = (
        (INCHES, 'Inches'),
        (NUMBER, '#'),
    )
    size_unit = models.CharField(
        max_length=4,
        choices=GOODS_ITEM_SIZE_UNITS,
        default=INCHES,
    )

    def __str__(self):
        if(self.size_unit == self.NUMBER):
            return "%s #%s" % (self.name, (self.size).normalize())
        else:
            return "%s %s\"" % (self.name, (self.size).normalize())


class FinishedGoodsItem(models.Model):
    date = models.DateField()
    goods_item = models.ForeignKey(GoodsItem, on_delete=models.CASCADE, related_name="finished_name")
    weight = models.DecimalField(max_digits=6, decimal_places=3)

    def __str__(self):
        return str(self.goods_item)


class SoldGoodsItem(models.Model):
    goods_item = models.ForeignKey(GoodsItem, on_delete=models.CASCADE, related_name="sold_name")
    date = models.DateField()
    weight = models.DecimalField(max_digits=6, decimal_places=3)

    def __str__(self):
        return str(self.goods_item)

And here is admin.py:

from django.contrib import admin
from django.db.models import Sum

from .models import GoodsItem, FinishedGoodsItem, SoldGoodsItem

@admin.register(SoldGoodsItem)
@admin.register(FinishedGoodsItem)
class FinishedGoodsItemAdmin(admin.ModelAdmin):
    fields = ('date', 'goods_item', 'weight')
    list_display = ('date', 'goods_item', 'weight')

@admin.register(GoodsItem)
class GoodsItemAdmin(admin.ModelAdmin):
    list_display = ('__str__', 'finished_good', 'sold_good', 'stock_available')

    def get_queryset(self, request):
        qs = super(GoodsItemAdmin, self).get_queryset(request)
        qs = qs.annotate(
            finished_good=Sum('finished_name__weight'),
            sold_good=Sum('sold_name__weight'),
            stock_available=Sum('finished_name__weight') - Sum('sold_name__weight'),
        )
        return qs

    def finished_good(self, obj):
        return obj.finished_good

    def sold_good(self, obj):
        return obj.sold_good

    def stock_available(self, obj):
        return obj.stock_available

In stock_available for each GoodsItem, I want to display the difference between all entries of FinishedGoodsItem and all entries of SoldGoodsItem. For now, I'm getting incorrect value for all three annotated fields which are finished_good, sold_good and stock_available. I'm unable to find the reason for that. In Django Debug Toolbar suggest that duplicate queries are being executed.

Answer 1

This is known issue and occurs when we try to combine multiple aggregation, as mentioned in docs.

As a workaround for this particular problem, we can use Subquery expression. Here is my updated admin.py using Subquery expression in get_queryset method of GoodsItemAdmin.

from django.contrib import admin
from django.db.models import Subquery, Sum, OuterRef

from .models import GoodsItem, FinishedGoodsItem, SoldGoodsItem

@admin.register(SoldGoodsItem)
class SoldGoodsItemAdmin(admin.ModelAdmin):
    fields = ('date', 'goods_item', 'weight')
    list_display = ('date', 'goods_item', 'weight')

@admin.register(FinishedGoodsItem)
class FinishedGoodsItemAdmin(admin.ModelAdmin):
    fields = ('date', 'goods_item', 'weight')
    list_display = ('date', 'goods_item', 'weight')

@admin.register(GoodsItem)
class GoodsItemAdmin(admin.ModelAdmin):
    list_display = ('__str__', 'finished_good', 'sold_good', 'stock_available')

    def get_queryset(self, request):
        qs = super(GoodsItemAdmin, self).get_queryset(request)
        qs = qs.annotate(
            finished_good = Subquery(FinishedGoodsItem.objects.filter(goods_item=OuterRef('pk'))\
                .values('goods_item_id').annotate(sum=Sum('weight')).values('sum')[:1]),
            sold_good = Subquery(SoldGoodsItem.objects.filter(goods_item=OuterRef('pk'))\
                .values('goods_item_id').annotate(sum=Sum('weight')).values('sum')[:1])
        )
        return qs

    def finished_good(self, obj):
        return obj.finished_good

    def sold_good(self, obj):
        return obj.sold_good

    def stock_available(self, obj):
        finished_good = 0 if self.finished_good(obj) is None else self.finished_good(obj)
        sold_good = 0 if self.sold_good(obj) is None else self.sold_good(obj)
        return '-' if (finished_good == 0 and sold_good == 0) else finished_good - sold_good

Hope someone finds this useful.