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python - Django Sum of Multiplied Values

in Technique[技术] by (71.8m points)

I am trying to show the total weight of assets I have presented on a table.

Each Asset has a weight and a quantity and I can present the total weight of each asset as below:

def by_item_weight(self):
    """This Method gives the total value of the asset. Weight * Qnty"""
    total = self.asset_quantity * self.asset_weight
    return total

What I would like to do is take the total item weight of each asset (result of qnty * weight) and present the over all weight of all assets.

I have tried to make various combinations of the below to return the sum of the by_item_weight 'column': 

Asset.objects.all().annotate(total=Sum('by_item_weight')

But I understand this will not work as by-item-weight is not defined in the model Asset.

I am presenting the table via a for loop as below on my assetslist.html template:

{% block content %}



<table class="well table table-striped text-center">
    <thead>
        <tr class="text-center">
            <th class="text-center">Asset ID:</th>
            <th class="text-center">Asset Name:</th>
            <th class="text-center">Asset Quantity:</th>
            <th class="text-center">Asset Weight / kg:</th>
            <th class="text-center">Total Weight / kg:</th>
            <th class="text-center">Asset Owner:</th>
        </tr>
    </thead>
    <tbody>
        <tr class="text-center">
{% for asset in object_list %}
            <td>{{ asset.id }}</td>
            <td>{{ asset.asset_name }}</td>
            <td>{{ asset.asset_quantity }}</td>
            <td>{{ asset.asset_weight }}</td>
            <td>{{ asset.by_item_weight }}</td>
            <td>{{ asset.asset_owner }}</td>


        </tr>


{% endfor %}

I am new to Python and am having trouble finding a way to capture the value of each asset.by_item_weight which If I could capture in a list I presume I could them sum the list and show the result.

My Models

class Asset(models.Model):
    asset_name = models.CharField(max_length=30)
    asset_quantity = models.IntegerField(default=0)
    asset_weight = models.IntegerField(default=0)
    asset_owner = models.ForeignKey(
        'AssetOwner',
        on_delete=models.CASCADE,
    ) # This should be a Foreign Key Drop down of AssetOwners owner_name.


    def by_item_weight(self):
        """This Method gives the total value of the asset. Weight * Qnty"""
        total = self.asset_quantity * self.asset_weight
        return total

    def __str__(self):
        return '{}'.format(self.asset_name)
        return 'Asset Quantity: {}'.format(self.asset_quantity)
        return 'Asset Weight: {}'.format(self.asset_weight)
        return 'Asset Owner: {}'.format(self.asset_owner)
        return 'Asset Owner: {}'.format(self.asset_owner)

Any help would be much appreciated.

UPDATED:

No Errors now, but still not able to show/display the value of sum_total

New Template

   {% extends "personal/header.html" %}

{% block content %}

<h1 class='text-center'>This is the full asset list not split by owner</h1></br>

    <table class="well table table-striped text-center">
        <thead>
            <tr class="text-center">
                <th class="text-center">Asset ID:</th>
                <th class="text-center">Asset Name:</th>
                <th class="text-center">Asset Quantity:</th>
                <th class="text-center">Asset Weight / kg:</th>
                <th class="text-center">Total Weight / kg:</th>
                <th class="text-center">Asset Owner:</th>
            </tr>
        </thead>
        <tbody>
            <tr class="text-center">
    {% for asset in object_list %}
                <td>{{ asset.id }}</td>
                <td>{{ asset.asset_name }}</td>
                <td>{{ asset.asset_quantity }}</td>
                <td>{{ asset.asset_weight }}</td>
                <td>{{ asset.by_item_weight }}</td>
                <td>{{ asset.asset_owner }}</td>


            </tr>


    {% endfor %}

        </tbody>
    </table>


<p class="" style="">Total Weight In Stock : {{ asset.sum_total }}</p>


<p class="text-center">{% include "sam/includes/backtosam.html" %}</p>



{% endblock %}

New Models

 from __future__ import unicode_literals
    from django.db import models
    from django.db.models import Sum, F, Count
    from django.db.models import Max
    from django.db.models import ExpressionWrapper
    from django.db.models import Aggregate

    class Asset(models.Model):
        asset_name = models.CharField(max_length=30)
        asset_quantity = models.IntegerField(default=0)
        asset_weight = models.IntegerField(default=0)
        asset_owner = models.ForeignKey(
            'AssetOwner',
            on_delete=models.CASCADE,
        ) # This should be a Foreign Key Drop down of AssetOwners owner_name.
    def by_item_weight(self):
        """This Method gives the total value of the asset. Weight * Qnty"""
        total = self.asset_quantity * self.asset_weight
        return total

    def sum_total(self):
        assets = Asset.objects.all().annotate(
        total_weight=ExpressionWrapper(F('asset_quantity') *     F('asset_weight'),output_field=IntegerField))
        the_sum = assets.aggregate(total=Sum('total_weight'))
        return the_sum

    def __str__(self):
        return '{}'.format(self.asset_name)
        return 'Asset Quantity: {}'.format(self.asset_quantity)
        return 'Asset Weight: {}'.format(self.asset_weight)
        return 'Asset Owner: {}'.format(self.asset_owner)
        return 'Asset Owner: {}'.format(self.asset_owner)
        return 'Total Weight of Assets: {}'.format(self.assets)

Updated view

from django.shortcuts import render
from django.http import HttpResponse
from django.core.cache import cache
from django.db.models import Sum, F

def get_total_weight(self):

    total_weight = cache.get('total_weight',-1)
    if total_weight == -1:
       total_weight = Asset.objects.annotate(total_weight=F('asset_quantity')*F('asset_weight')).aggregate(total=Sum('total_weight'))
       # this is tested
       cache.set('total_weight',total_weight)
       return total_weight

def index(request):

    return render(request, 'sam/index.html')

def assetslist(request):

    return render(request,'sam/assetslist.html',{'total_weight': get_total_weight}, assets = Asset.objects.all())

I suspect there is an issue with the above assetslist method which I am not apparently calling.

Template

{% extends "personal/header.html" %}

{% block content %}

<h1 class='text-center'>This is the full asset list not split by owner</h1></br>

    <table class="well table table-striped text-center">
        <thead>
            <tr class="text-center">
                <th class="text-center">Asset ID:</th>
                <th class="text-center">Asset Name:</th>
                <th class="text-center">Asset Quantity:</th>
                <th class="text-center">Asset Weight / kg:</th>
                <th class="text-center">Total Weight / kg:</th>
                <th class="text-center">Asset Owner:</th>
            </tr>
        </thead>
        <tbody>
            <tr class="text-center">
    {% for asset in object_list %}
                <td>{{ asset.id }}</td>
                <td>{{ asset.asset_name }}</td>
                <td>{{ asset.asset_quantity }}</td>
                <td>{{ asset.asset_weight }}</td>
                <td>{{ asset.by_item_weight }}</td>
                <td>{{ asset.asset_owner }}</td>



            </tr>


    {% endfor %}

        </tbody>
    </table>
<p class="" style="">Total Weight In Stock : {{ get_total_weight }}</p>
<p class="" style="">Total Weight In Stock : {{ assetslist }}</p>



    <!-- <table class="well table table-striped text-center">
        <thead>
            <tr class="text-center">

                <th class="text-center"></th>
                <th class="text-center"></th>
                <th class="text-center"></th>
                <th class="text-center"></th>
                <th class="text-center">Total Weight / kg:</th>

            </tr>
        </thead>
        <tbody>
            <tr class="text-center">
    {% for sum_weight in object_list %}

                <td></td>
                <td></td>
                <td></td>
                <td></td>
                <td>{{ asset.sum_total }}</td>

            </tr>


    {% endfor %}
        </tbody>
    </table> -->






<p class="text-center">{% include "sam/includes/backtosam.html" %}</p>



{% endblock %}

Thanks for any input/suggestions.

Further UPDATE:

I have adjusted the view to the below: 

from django.core.cache import cache
from django.db.models import Sum, F


def get_total_weight(self):
    total_weight = cache.get('total_weight',-1)
    if total_weight == -1:
       total_weight = Asset.objects.annotate(total_weight=F('asset_quantity')*F('asset_weight')).aggregate(total=Sum('total_weight'))
       # this is tested
       cache.set('total_weight',total_weight)

    return total_weight

    render(request,'template_name',{'total_weight': get_total_weight, assets = Asset.objects.all() } )

I am getting errors on the assets = Asset.objects.all() } ) = sign. SyntaxError: invalid syntax

I presume that render needs to be in its own function?

UPDATE:

I have updated my views and moved the def from the models.py

my views.py file is as below 

    def total_weight(request):

    assets = Asset.objects.all().annotate(
    total_weight=ExpressionWrapper(F('asset_quantity') * F('asset_weight'), 
                                   output_field=IntegerField() ))

    return render(request, 'sam/index.html')


def sum_total(request):

    the_total = assets.aggregate(total=Sum('total_weight'))

    return render(request, 'sam/assetlist.html')

def index(request):

    return render(request, 'sam/index.html')

def by_item_weight(self):
        """This Method gives the total value of the asset. Weight * Qnty"""
        total = self.asset_quantity * self.asset_weight
        return total

    def get_total_weight(self):
        total_weight = Asset.objects.filter(by_item_weight__isnull=True).aggregate(Sum('by_item_weight'))
        Asset.objects.annotate(total_weight=F('asset_quantity')*F('asset_weight')).aggregate(total=Sum('total_weight'))


    def __str__(self):
        return '{}'.format(self.asset_name)
        return '{}'.format(self.total_weight

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1 Answer

by (71.8m points)

You could use the ExpressionWrapper() (mostly Django 1.8+)

assets = Asset.objects.all().annotate(
    total_weight=ExpressionWrapper(F('asset_quantity') * F('asset_weight'), 
                                   output_field=IntegerField() ))

That should give you the total weight for each object, i.e. quantity times weight.

Now, you should be able to get a sum from all the total_weights.

Edit: Now you can use Aggregate to get the total

assets.aggregate(total=Sum('total_weight'))

{'total': 1234.5678}

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