Primary key behavior in Django - Step-by-Step Execution

Concept Flow - Primary key behavior

Define Model

↓

Primary Key Field?

↓

Use defined

↓

Save instance

→Assign PK value

↓

PK must be unique

↓

Instance saved in DB

This flow shows how Django handles primary keys: if you define one, it uses that; if not, it adds an 'id' field automatically. The primary key must be unique for each saved instance.

Execution Sample

Django

class Book(models.Model):
    title = models.CharField(max_length=100)

book = Book(title='Django Basics')
book.save()

Defines a model without a primary key field, so Django adds an 'id' automatically when saving.

Execution Table

Step	Action	Primary Key Field	PK Value Before Save	PK Value After Save	DB State
1	Define Book model	No explicit PK	None	None	Model ready, no data
2	Create Book instance	No explicit PK	None	None	Instance created, unsaved
3	Call save()	No explicit PK	None	1 (auto-assigned)	Book with id=1 saved
4	Create another Book instance	No explicit PK	None	None	Second instance created, unsaved
5	Call save() on second instance	No explicit PK	None	2 (auto-assigned)	Book with id=2 saved
6	Define model with explicit PK	Yes, e.g. isbn	User value	User value	Instance saved with user PK
7	Try saving duplicate PK	Yes	Existing PK	Error	Save fails due to PK uniqueness
8	End	-	-	-	No further saves

💡 Execution stops because all instances are saved or error occurs on duplicate PK

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 6	After Step 7	Final
Book.id	None	1	2	-	-	2
Book.isbn	-	-	-	User set	Error on duplicate	User set or error
DB entries	Empty	1 entry (id=1)	2 entries (id=1,2)	Entry with user PK	No new entry on error	2 or error

Key Moments - 3 Insights

Why does Django add an 'id' field if I don't define a primary key?

Django requires a primary key for each model to uniquely identify records. If you don't define one, it automatically adds an 'id' field as an integer primary key. See execution_table rows 1-3.

What happens if I try to save two instances with the same primary key value?

The database enforces uniqueness on primary keys. Saving a duplicate primary key causes an error and the save fails. See execution_table row 7.

Can I use a field other than 'id' as the primary key?

Yes, you can define any field as primary key by setting primary_key=True. Django then uses that field instead of adding 'id'. See execution_table row 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the primary key value after saving the first Book instance?

A 1

B None

C 0

D Auto-generated string

💡 Hint

Check execution_table row 3 under 'PK Value After Save'

At which step does Django assign a primary key automatically?

A Step 1

B Step 3

C Step 2

D Step 6

💡 Hint

Look at when 'PK Value After Save' changes from None to a number in execution_table

If you define a field as primary key, what happens to the 'id' field?

A It becomes a foreign key

B It is still added automatically

C It is removed and not added

D It duplicates the primary key field

💡 Hint

Refer to concept_flow where 'Primary Key Field?' decision leads to using defined PK or adding 'id'

Concept Snapshot

Django models require a primary key (PK) to uniquely identify records.
If no PK is defined, Django adds an 'id' field automatically.
You can define any field as PK by setting primary_key=True.
PK values must be unique; duplicates cause save errors.
PK is assigned when saving the instance to the database.

Full Transcript

In Django, every model needs a primary key to uniquely identify each record. If you don't define one, Django automatically adds an 'id' field as an integer primary key. When you create and save a model instance without a defined primary key, Django assigns a unique 'id' value starting from 1. If you define a field as primary key by setting primary_key=True, Django uses that field instead and does not add the 'id' field. The primary key value must be unique for each record; trying to save a duplicate primary key causes an error and the save fails. This behavior ensures each record can be uniquely retrieved and managed in the database.