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Saliha Qaisar
Saliha Qaisar

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Django Many-to-Many Relationships: The Hidden PostgreSQL Sequence Desynchronisation Problem After Database Restores

#django #postgres #python #migrations

The Mystery That Nearly Drove Me Crazy

Imagine this : Your Django application works flawlessly without any sorts of difficulty locally, but after you deploy it to staging and UAT( Pre-Production) environments, something very strange happens. Some of Many-to-many relationships just stop working. Forms submit as expected, data appears to be populated, and admin panel operations seem to finish without any error glitch or a hitch.
But , some of the M2M relationships refuse to be saved. No error messages, no obvious issues , just data silently disappearing in thin air leaving you baffled and bamboozled.
This was exactly the nightmare scenario I found myself in recently, and the solution turned out to be far more subtle than I initially imagined.

The Setup:

I had a Django application with four models: Employee, SegregationType, Item, and Skill. Three of these models (Employee, SegregationType, and Item) had many-to-many relationships with Skill:

class Employee(models.Model):
    name = models.CharField(max_length=100)
    skills = models.ManyToManyField('Skill', blank=True)

class SegregationType(models.Model):
    name = models.CharField(max_length=100)
    skills = models.ManyToManyField('Skill', blank=True)

class Item(models.Model):
    name = models.CharField(max_length=100)
    skills = models.ManyToManyField('Skill', blank=True)

class Skill(models.Model):
    name = models.CharField(max_length=100)
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Problem Emerges

Everything worked flawlessly in my local development environment. But in staging and UAT(Pre-Production), attempting to assign skills to SegregationType or Item instances would:

  • Return success responses from the API.
  • Show no errors in the Django admin panel.
  • But fail to actually create the relationships.

Initial Suspicions

My first instinct was to blame the frontend. Maybe there was an issue with payload encryption or data serialization? After thorough investigation, the frontend was innocent.

Network and Concurrency Theories

Next, I suspected network issues—perhaps packet loss was causing incomplete requests. I also considered race conditions: maybe the main model transaction wasn't committed before Django tried to create the M2M relationships. Both theories proved incorrect.

The Real Culprit Emerges

The breakthrough came when I remembered that a few days prior, we had populated staging and UAT databases using a backup restore. During subsequent operations, we started encountering this error:

django.db.utils.IntegrityError: duplicate key value violates unique constraint "django_migrations_pkey"
DETAIL: Key (id)=(3) already exists.
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This was the smoking gun that led me to the real issue: PostgreSQL sequence desynchronization.

Understanding PostgreSQL Sequences

When you restore data into PostgreSQL from a backup, the records themselves are inserted correctly, but the sequences (which control the next value for auto-incrementing primary keys) aren’t automatically updated (Sadly!). This causes a subtle but potentially destructive mismatch:

  • Your table could have rows with IDs up to 10000s.
  • However, the sequence still assumes the next available ID is 1, as if the table were empty.
  • Now, when Django tries to insert a new record, it attempts to use ID=1, which already exists, triggering a primary key conflict.

The Initial Fix

I had resolved the main model sequence issues using this script:

from django.db import connection
from django.apps import apps

with connection.cursor() as cursor:
    for model in apps.get_models():
        table = model._meta.db_table
        pk_field = model._meta.pk

        if not pk_field.auto_created:
            continue  # Skip if no auto-created PK

        sequence_sql = f"SELECT pg_get_serial_sequence('{table}', '{pk_field.column}');"
        cursor.execute(sequence_sql)
        result = cursor.fetchone()

        if result and result[0]:
            sequence_name = result[0]
            update_sql = f"SELECT setval('{sequence_name}', (SELECT COALESCE(MAX({pk_field.column}), 1) FROM {table}) + 1, false);"
            print(f"✅ Resetting sequence for {table} using {sequence_name}")
            cursor.execute(update_sql)
        else:
            print(f"⚠️ No sequence found for table: {table}")
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The Actual Problem at hand : Implicit Through Tables

The key realization that took me hours to uncover: Django’s implicit many-to-many through tables also have auto-incrementing ID fields with their own sequences. Plus, apps.get_models() doesn’t return these implicit through tables.

When you define a ManyToManyField without specifying a through model, Django automatically creates a through table with:

  • An auto-incrementing id field
  • Foreign keys to both related models
  • Its own PostgreSQL sequence

These implicit through table sequences were also desynchronized after the database restore!

The Complete Solution

To fix the M2M relationships, I had to manually reset the sequences for all implicit through tables:

-- Example for SegregationType skills M2M relationship
SELECT setval(
    pg_get_serial_sequence('"jobs_segregationtype_skills"', 'id'),
    COALESCE(MAX(id), 1),
    true
) FROM "jobs_segregationtype_skills";
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You can discover the through table names programmatically:

# Get the through table name for a M2M field
through_table = SegregationType.skills.through._meta.db_table
print(f"Through table: {through_table}")
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A Comprehensive Django Management Command

Here's a complete solution as a Django management command:

from django.core.management.base import BaseCommand
from django.db import connection
from django.apps import apps

class Command(BaseCommand):
    help = 'Reset PostgreSQL sequences for all models and M2M through tables'

    def handle(self, *args, **options):
        with connection.cursor() as cursor:
            # Reset sequences for main models
            for model in apps.get_models():
                self.reset_model_sequence(cursor, model)

                # Reset sequences for M2M through tables
                for field in model._meta.get_fields():
                    if field.many_to_many and not field.remote_field.through._meta.auto_created:
                        continue  # Skip explicit through models
                    elif field.many_to_many:
                        self.reset_model_sequence(cursor, field.remote_field.through)

    def reset_model_sequence(self, cursor, model):
        table = model._meta.db_table
        pk_field = model._meta.pk

        if not pk_field.auto_created:
            return

        sequence_sql = f"SELECT pg_get_serial_sequence('{table}', '{pk_field.column}');"
        cursor.execute(sequence_sql)
        result = cursor.fetchone()

        if result and result[0]:
            sequence_name = result[0]
            update_sql = f"SELECT setval('{sequence_name}', (SELECT COALESCE(MAX({pk_field.column}), 1) FROM {table}) + 1, false);"
            self.stdout.write(f"✅ Resetting sequence for {table}")
            cursor.execute(update_sql)
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Prevention Strategies

  1. Proper Database Dumps: When creating backups, ensure sequences are included and properly handled
  2. Post-Restore Checklist: Always run sequence reset commands after database restores
  3. Automated Testing: Include M2M relationship tests in your deployment verification suite
  4. Monitoring: Set up alerts for IntegrityError exceptions in production

Key Takeaways

  • Silent failures are the worst: M2M relationship failures can be completely silent, making them incredibly difficult to debug
  • Implicit complexity: Django's implicit through tables add hidden complexity that's easy to overlook
  • Environment parity matters: Issues that don't reproduce locally can be the most challenging to solve
  • Sequence management: PostgreSQL sequence desynchronization is a common but often overlooked issue after database restores

Conclusion

This debugging challenge sharpened my engineering skills and taught me valuable lessons about Django and PostgreSQL.
The issue stemmed from Django's many-to-many relationships, which use implicit through tables with auto-incrementing IDs and sequences. If these sequences aren't properly managed, they can cause data conflicts and silent failures.
This experience underscored the importance of understanding Django's database interactions and the need for careful sequence management in PostgreSQL to maintain relationship integrity.

Crucially, whenever you restore data in a Django application, you must examine and reset sequences for through tables—not just your main model tables.

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