pre_save and post_save signals in Django - Step-by-Step Execution

Concept Flow - pre_save and post_save signals

Model instance about to save

↓

pre_save signal triggered

↓

Instance saved to DB

↓

post_save signal triggered

↓

Further actions or cleanup

When saving a model, Django first triggers pre_save signal before saving, then saves the instance, and finally triggers post_save signal after saving.

Execution Sample

Django

from django.db.models.signals import pre_save, post_save
from django.dispatch import receiver

@receiver(pre_save, sender=MyModel)
def before_save(sender, instance, **kwargs):
    print('Before saving:', instance)

@receiver(post_save, sender=MyModel)
def after_save(sender, instance, created, **kwargs):
    print('After saving:', instance, 'Created:', created)

This code listens for pre_save and post_save signals on MyModel and prints messages before and after saving.

Execution Table

Step	Signal/Event	Instance State	Action	Output
1	Call save() on instance	Instance with data	Trigger pre_save signal	Print 'Before saving: <instance>'
2	pre_save signal	Instance with data	Run before_save handler	Print message before DB save
3	Save instance to DB	Instance saved	Database write operation	No output
4	Trigger post_save signal	Instance saved	Run after_save handler	Print 'After saving: <instance> Created: True/False'
5	End of save()	Instance saved	Return from save()	No output

💡 Save process completes after post_save signal handler runs

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
instance	New unsaved object	Ready to save	Saved in DB	Saved in DB	Saved in DB
created	N/A	N/A	N/A	True if new, False if update	True/False

Key Moments - 3 Insights

Why does pre_save run before the instance is saved to the database?

Because pre_save signal is designed to let you modify or check the instance before it is saved, as shown in execution_table step 2.

What does the 'created' parameter in post_save indicate?

'created' is True if the instance was newly created and saved, False if it was an update, as shown in execution_table step 4.

Can changes made in pre_save affect what is saved to the database?

Yes, because pre_save runs before saving, any changes to the instance in pre_save will be saved, as implied in the flow and execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the instance actually saved to the database?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Check the 'Action' column for the database write operation in execution_table row 3.

According to variable_tracker, what is the value of 'created' after step 4?

A Always True

B Always False

C True if new instance, False if update

D Undefined

💡 Hint

Look at the 'created' row in variable_tracker after step 4.

If you modify the instance in pre_save, when will those changes be saved?

A Immediately after pre_save signal finishes

B During the save operation in step 3

C After post_save signal

D They won't be saved

💡 Hint

Refer to concept_flow and execution_table steps 2 and 3 about when saving happens.

Concept Snapshot

Django signals pre_save and post_save run around model saving.
pre_save runs before saving, allowing changes before DB write.
post_save runs after saving, useful for actions after save.
Use @receiver decorator to connect handlers.
'created' in post_save shows if instance is new or updated.

Full Transcript

When you save a Django model instance, the pre_save signal runs first. This lets you check or change the instance before it goes into the database. Then Django saves the instance to the database. After saving, the post_save signal runs. It tells you the save is done and whether the instance was new or updated. You connect functions to these signals using the @receiver decorator. This way, you can run code automatically before or after saving a model.