django admin list_display from two tables, no relations like foreign key or manytomany relation available

Question

i have two tables, one is MeasuredController and MeasuredGrid, and there is no relation like foreign key or manytoman etc , and in admin i have to show two fields of MeasuredGrid i.e power and status, where MeasuredController's senddate = MeasuredGrid's senddate, in two different column, i have wrote code like below, but in the current code , the database will be hit two time for each object, so is there a way like select related or use cache concept ?

list_display = ("grid_status", "grid_power")

def grid_status(self, obj):
    STATUS_CHOICES = {0:"Outage", 1:"No Outage" }
    mobj = MeasuredGrid.objects.filter(senddate=obj.senddate).latest("senddate")

    try:
        return STATUS_CHOICES[int(mobj.status)], 2
    except:
        pass
grid_status.short_description = 'Grid Status'

def grid_power(self, obj):
    mobj = MeasuredGrid.objects.filter(senddate=obj.senddate).latest("senddate")
    return mobj.power

grid_power.short_description = 'Grid Power[W]'

Answer 1

You can use Cache Framework. It is very easy:

# coding: utf-8
from django.core.cache import cache
from django.contrib import admin

from .models import MeasuredController, MeasuredGrid


class MeasuredControllerAdmin(admin.ModelAdmin):
    list_display = ("grid_status", "grid_power")

    STATUS_CHOICES = {
        0: "Outage",
        1: "No Outage",
    }

    def grid_status(self, obj):
        mobj = self._get_mobj_data(obj)
        return mobj['status']
    grid_status.short_description = 'Grid Status'

    def grid_power(self, obj):
        mobj = self._get_mobj_data(obj)
        return mobj['power']
    grid_power.short_description = 'Grid Power[W]'

    def _get_mobj_data(self, obj):
        """Get a relevant MeasuredGrid object for a given MeasuredController"""

        data = cache.get('mobj_%s' % obj.pk)
        if data is not None:
            return data

        mobj = MeasuredGrid.objects.filter(senddate=obj.senddate).latest("senddate")

        status = None
        try:
            status = self.STATUS_CHOICES[int(mobj.status)], 2
        except: # <---------- Not the best decision. You probably need ValueError or KeyError
            pass

        data = {
            "id": mobj.pk,
            "power": mobj.power,
            "status": status,
        }

        cache.set('mobj_%s' % obj.pk, data) # the default timeout is 300 seconds

        return data

NB: the default cache backend is django.core.cache.backends.locmem.LocMemCache, so the cache will work even in dev environment (ie. DEBUG = True).