Too many SQL queries with Django Admin many-to-many inline view

Question

I have a 'diamond' relationship between four models. AlignedScan is basically a pay-load carrying model facilitating a many-to-many relationship between Scan and Alignment. Both Scan and Alignment have a to-one relationship to Scan Project. When trying to display a inline-list of AlignedScans in an Alignment detail window (in Django Admin), it works but is very slow due to two extra SQL queries being executed for each record. What am I doing wrong?

Note: the inline view is meant to be read-only. Records are created exclusively via scripted imports.

These are my models:

from django.contrib.gis.db import models
from django.db.models import Count, Q

class ScanProject_Manager(models.Manager):

    def get_queryset(self):
        return (super().get_queryset()
                    .annotate(number_of_alignments=Count('alignments', distinct=True))
                    .annotate(number_of_usedscans=Count('scans', distinct=True, filter=Q(scans__used=True)))
        )

class ScanProject(models.Model):
    objects = ScanProject_Manager()


    slug = models.CharField(max_length=30,primary_key=True,) 
    label = models.CharField(max_length=100,)

    #...

class Scan(models.Model):
    name = models.CharField(max_length=24,)
    #...

    scanproject = models.ForeignKey(
        ScanProject,
        related_name='scans',
        on_delete=models.CASCADE,
    )

class Alignment(models.Model):
    aligned_when = models.CharField(max_length=13,)
    #...

    scanproject = models.ForeignKey(
        ScanProject,
        related_name='alignments',
        on_delete=models.CASCADE,
        )

class AlignedScan(models.Model):
    registered = models.BooleanField()
    x = models.FloatField(blank=True, null=True,)
    y = models.FloatField(blank=True, null=True,)
    z = models.FloatField(blank=True, null=True,)
    #...

    alignment = models.ForeignKey(
        Alignment,
        related_name='alignedscans',
        on_delete=models.CASCADE, \
        )
    
    scan = models.ForeignKey(
        Scan, 
        related_name='alignedscans', 
        on_delete=models.CASCADE,
        )

class AlignedScan_Manager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().select_related('scan')

I've tried with and without the AlignedScan_Manager, I've tried using a plain-vanilla straight out of the box InlineModel:

class AlignedScan_Inline(admin.TabularInline):
    model = AlignedScan

and i've tried a custom one:

class AlignedScan_Inline(admin.TabularInline):
    verbose_name = 'Scan'    # omit 'Aligned' from the UI, as it is implied
    verbose_name_plural = 'Scans'

    model = AlignedScan 
    fk_name = 'alignment'

    fields = ('registered', 'x', 'y', 'z')
    readonly_fields = ('registered', 'x', 'y', 'z') 
    extra = 0
    show_change_link = True  
    list_select_related = ['scan']  # tried with and without this


    def has_add_permission(self, request, owner):
        return False
    def has_change_permission(self, request, owner):
        return True #TBC: respect user permissions
    def has_delete_permission(self, request, owner):
        return False  #TBC: respect user permissions

The final version must also show fields from the Scan table, but even when I don't, the problem persists.

I get two SQL queries for each record:

SELECT "scans_scan"."id",
       "scans_scan"."name",
        -- ....

       "scans_scan"."scanproject_id"
 FROM "scans_scan"
 WHERE "scans_scan"."id" = 3413
 LIMIT 21 10 similar queries.   

and

SELECT "scans_alignment"."id",
        "scans_alignment"."aligned_when",
        "scans_alignment"."scanproject_id",
        -- ...
        "scans_alignment"."imported_by_id"
    FROM "scans_alignment"
    WHERE "scans_alignment"."id" = 4
    LIMIT 21 11 similar queries.  Duplicated 11 times.

Answer 1

Try to select the related fields from get_queryset:

class AlignedScan_Inline(admin.TabularInline):
    ...
    def get_queryset(self, *args, **kwargs):
        return super().get_queryset(*args, **kwargs).select_related('scan', 'alignment')