Primary key behavior in Django - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a primary key in Django models?

A primary key is a unique identifier for each record in a database table. In Django, it uniquely identifies each model instance.

beginner

What happens if you don't define a primary key in a Django model?

Django automatically adds an id field as an auto-incrementing primary key.

intermediate

Can you use a field other than id as a primary key in Django?

Yes, you can set any field as the primary key by adding primary_key=True to that field.

intermediate

What is the behavior of a primary key field with primary_key=True in Django?

It becomes unique, non-nullable, and automatically used as the identifier for model instances.

beginner

Why should a primary key be unique and non-null in Django models?

Because it identifies each record uniquely and must always have a value to avoid confusion in data retrieval.

What does Django do if no primary key is defined in a model?

A Throws an error

B Adds an auto-incrementing <code>id</code> field as primary key

C Uses the first field as primary key

D Creates a composite key automatically

Django automatically adds an id field as the primary key if none is defined.

Which of these is true about a primary key field in Django?

A It must be unique

B It can be null

C It can have duplicate values

D It is optional

Primary keys must be unique and non-null to identify records uniquely.

How do you set a custom field as a primary key in Django?

A Add <code>unique=True</code> to the field

B Set <code>auto_increment=True</code>

C Add <code>primary_key=True</code> to the field

D Rename the field to <code>id</code>

Setting primary_key=True on a field makes it the primary key.

What type of field does Django use by default for the primary key?

A BooleanField

B CharField

C ForeignKey

D AutoField

Django uses an AutoField, an integer that auto-increments, as the default primary key.

Why is it important that a primary key is non-null?

A To ensure every record can be uniquely identified

B To speed up queries

C To allow multiple records with the same key

D To allow null values in other fields

A non-null primary key ensures each record is uniquely identifiable.

Explain what a primary key is in Django and how Django handles it if you don't define one.

Think about what Django adds automatically to every model.

Describe how to set a custom primary key in a Django model and what changes this causes.

Focus on the field attribute that changes primary key behavior.

Practice

1. What does Django do if you do not specify a primary key in your model?

easy

A. Uses the first field defined as the primary key.

B. Automatically adds an id field as the primary key.

C. Raises an error and refuses to create the model.

D. Requires you to manually add a primary key later.

Solution

1. Step 1: Understand Django's default behavior

   Django automatically adds an id field as a primary key if none is specified.

2. Step 2: Confirm no error or manual step needed

   No error is raised and no manual primary key is required by default.

3. Final Answer:

   Automatically adds an id field as the primary key. -> Option B

4. Quick Check:

   Default primary key = id field [OK]

Hint: Remember: Django adds id automatically if no primary key [OK]

Common Mistakes:

• Thinking Django raises an error without a primary key
• Assuming first field is primary key by default
• Believing manual primary key is always required

2. Which of the following is the correct way to set a field as the primary key in a Django model?

easy

A. name = models.CharField(max_length=100, primary_key=True)

B. name = models.CharField(max_length=100, key_primary=True)

C. name = models.CharField(max_length=100, primary=True)

D. name = models.CharField(max_length=100, is_primary=True)

Solution

1. Step 1: Identify correct keyword argument

   The correct argument to set a primary key is primary_key=True.

2. Step 2: Match with options

   Only name = models.CharField(max_length=100, primary_key=True) uses primary_key=True correctly.

3. Final Answer:

   name = models.CharField(max_length=100, primary_key=True) -> Option A

4. Quick Check:

   Use primary_key=True to set primary key [OK]

Hint: Use primary_key=True exactly to set primary key [OK]

Common Mistakes:

• Using incorrect argument names like primary=True
• Confusing primary_key with other flags
• Misspelling primary_key

3. Given this model:

class Product(models.Model):
    sku = models.CharField(max_length=20, primary_key=True)
    name = models.CharField(max_length=100)

p = Product(sku='X123', name='Widget')
p.save()
print(Product.objects.get(pk='X123').name)

What will be printed?

medium

A. None

B. X123

C. Error: No primary key field found

D. Widget

Solution

1. Step 1: Understand primary key usage

   The sku field is set as primary key, so pk='X123' looks up by sku.

2. Step 2: Trace the query result

   The object with sku 'X123' has name 'Widget', so printing .name outputs 'Widget'.

3. Final Answer:

   Widget -> Option D

4. Quick Check:

   Primary key lookup returns object name [OK]

Hint: pk uses the primary key field, here sku, to find object [OK]

Common Mistakes:

• Assuming pk means id field always
• Expecting error because id field missing
• Confusing printed value with primary key value

4. What is wrong with this model definition?

class Customer(models.Model):
    id = models.IntegerField(primary_key=True)
    id = models.CharField(max_length=10, primary_key=True)

medium

A. You must use AutoField for primary keys, not IntegerField or CharField.

B. Primary keys cannot be named 'id'.

C. You cannot have two fields with primary_key=True in the same model.

D. The model is missing a Meta class to define primary key.

Solution

1. Step 1: Check primary key uniqueness in model

   A Django model can have only one primary key field.

2. Step 2: Identify duplicate primary_key=True fields

   Here, two fields named 'id' are declared with primary_key=True, which is invalid.

3. Final Answer:

   You cannot have two fields with primary_key=True in the same model. -> Option C

4. Quick Check:

   Only one primary key allowed per model [OK]

Hint: Only one field can have primary_key=True in a model [OK]

Common Mistakes:

• Thinking multiple primary keys are allowed
• Believing primary key must be AutoField
• Assuming primary key name restrictions

5. You want to create a Django model where the email field uniquely identifies each user and acts as the primary key. Which is the best way to define this model?

hard

A. class User(models.Model):
email = models.EmailField(primary_key=True)
name = models.CharField(max_length=100)

B. class User(models.Model):
email = models.EmailField(unique=True)
name = models.CharField(max_length=100)

C. class User(models.Model):
id = models.AutoField(primary_key=True)
email = models.EmailField(unique=True)
name = models.CharField(max_length=100)

D. class User(models.Model):
email = models.EmailField()
name = models.CharField(max_length=100)
class Meta:
primary_key = 'email'

Solution

1. Step 1: Understand primary key vs unique

   Primary key uniquely identifies records and is used as the main identifier.

2. Step 2: Check options for setting email as primary key

   class User(models.Model):
       email = models.EmailField(primary_key=True)
       name = models.CharField(max_length=100)

   sets email as primary key directly, which is correct.

   class User(models.Model):
       email = models.EmailField(unique=True)
       name = models.CharField(max_length=100)

   only makes it unique but not primary key.

   class User(models.Model):
       id = models.AutoField(primary_key=True)
       email = models.EmailField(unique=True)
       name = models.CharField(max_length=100)

   keeps default id as primary key.

   class User(models.Model):
       email = models.EmailField()
       name = models.CharField(max_length=100)
       class Meta:
           primary_key = 'email'

   uses invalid Meta syntax.

3. Final Answer:

   Define email with primary_key=True in the model field. -> Option A

4. Quick Check:

   Set primary_key=True on email field to make it primary key [OK]

Hint: Use primary_key=True on email field to make it primary key [OK]

Common Mistakes:

• Using unique=True instead of primary_key=True
• Keeping default id field as primary key
• Trying to set primary key in Meta class