get() for single objects in Django - Mini Project: Build & Apply

Using Django's get() to Retrieve a Single Object

📖 Scenario: You are building a simple Django app to manage a library. You want to find a specific book by its exact title from the database.

🎯 Goal: Learn how to use Django's get() method to retrieve a single object from the database by a unique field.

📋 What You'll Learn

Create and save a Django model instance representing a book with a title and author.

Set up a variable with the exact title to search for.

Use Django's get() method to retrieve the book with the matching title.

Handle the final step to confirm the retrieval in the code structure.

💡 Why This Matters

🌍 Real World

Finding a specific record in a database by a unique identifier is common in web apps, like looking up a user by email or a product by SKU.

💼 Career

Understanding how to retrieve single objects with get() is essential for backend developers working with Django to build efficient and precise data queries.

1

Create a Book instance

Create and save a Django model instance called book with title set to 'Django Basics' and author set to 'Alice'.

Django

# Create and save a Book instance with title 'Django Basics' and author 'Alice'
# Your code here

Hint

Use book = Book(title='Django Basics', author='Alice'); book.save() to create and save the instance to the database.

2

Set the search title variable

Create a variable called search_title and set it to the string 'Django Basics'.

Django

book = Book(title='Django Basics', author='Alice')
book.save()
# Create a variable search_title with value 'Django Basics'
# Your code here

Hint

Just assign the string 'Django Basics' to search_title.

3

Retrieve the book using get()

Use Django's Book.objects.get() method with title=search_title to retrieve the book. Store the result in a variable called found_book.

Django

book = Book(title='Django Basics', author='Alice')
book.save()
search_title = 'Django Basics'
# Use Book.objects.get(title=search_title) and assign to found_book
# Your code here

Hint

Use Book.objects.get(title=search_title) to find the book.

4

Confirm retrieval with a final statement

Add a line that assigns found_book.title to a variable called result_title to confirm the book was retrieved.

Django

book = Book(title='Django Basics', author='Alice')
book.save()
search_title = 'Django Basics'
found_book = Book.objects.get(title=search_title)
# Assign found_book.title to result_title
# Your code here

Hint

Assign found_book.title to result_title.

Practice

1. What does the Django get() method do when used on a model's manager?

easy

A. It updates the object matching the filter.

B. It returns a list of all objects matching the filter.

C. It returns exactly one object matching the filter or raises an error.

D. It deletes the object matching the filter.

Solution

1. Step 1: Understand the purpose of get()

   The get() method is designed to find exactly one object that matches the given filter criteria.

2. Step 2: Recognize behavior on multiple or zero matches

   If no objects or more than one object match, get() raises an error instead of returning multiple objects or none.

3. Final Answer:

   It returns exactly one object matching the filter or raises an error. -> Option C

4. Quick Check:

   get() returns one object or error [OK]

Hint: Remember: get() expects one object, else error [OK]

Common Mistakes:

• Thinking get() returns multiple objects like filter()
• Assuming get() updates or deletes objects
• Expecting get() to return None if no match

2. Which of the following is the correct syntax to get a single Book object with id=5 using Django ORM?

easy

A. Book.objects.get(id=5)

B. Book.get.objects(id=5)

C. Book.objects.filter(id=5)

D. Book.objects.get.filter(id=5)

Solution

1. Step 1: Recall correct method call order

   In Django ORM, get() is called on the model manager accessed by objects.

2. Step 2: Verify syntax correctness

   The correct syntax is Book.objects.get(id=5). Other options misuse method chaining or order.

3. Final Answer:

   Book.objects.get(id=5) -> Option A

4. Quick Check:

   Correct syntax = Book.objects.get(id=5) [OK]

Hint: Use Model.objects.get(field=value) syntax [OK]

Common Mistakes:

• Swapping method and manager order
• Calling get() after filter() incorrectly
• Using get() as an attribute instead of method

3. Given the model Author with two entries having name='Alice', what happens when you run Author.objects.get(name='Alice')?

medium

A. Returns the first Author object with name 'Alice'.

B. Returns a list of Author objects with name 'Alice'.

C. Raises a DoesNotExist exception.

D. Raises a MultipleObjectsReturned exception.

Solution

1. Step 1: Understand get() behavior with multiple matches

   If more than one object matches the filter, get() raises a MultipleObjectsReturned exception.

2. Step 2: Apply to given scenario

   Since two Authors have name='Alice', calling get(name='Alice') triggers this exception.

3. Final Answer:

   Raises a MultipleObjectsReturned exception. -> Option D

4. Quick Check:

   Multiple matches cause MultipleObjectsReturned [OK]

Hint: Multiple matches with get() cause MultipleObjectsReturned error [OK]

Common Mistakes:

• Assuming get() returns first match silently
• Expecting get() to return a list
• Confusing DoesNotExist with multiple matches

4. What is wrong with this code snippet?

user = User.objects.get(username='john')
print(user.email)

Assuming no user with username 'john' exists.

medium

A. It will print an empty string for email.

B. It will raise a DoesNotExist exception.

C. It will raise a MultipleObjectsReturned exception.

D. It will return None and cause AttributeError on print.

Solution

1. Step 1: Check behavior when no object matches get()

   If no object matches the filter, get() raises a DoesNotExist exception.

2. Step 2: Apply to given code

   Since no user with username 'john' exists, User.objects.get(username='john') raises DoesNotExist before print runs.

3. Final Answer:

   It will raise a DoesNotExist exception. -> Option B

4. Quick Check:

   No match with get() causes DoesNotExist [OK]

Hint: No match with get() raises DoesNotExist error [OK]

Common Mistakes:

• Expecting get() to return None if no match
• Assuming print runs without error
• Confusing DoesNotExist with MultipleObjectsReturned

5. You want to get a single Product with sku='12345'. If it doesn't exist, you want to create it with name='New Product'. Which code correctly does this using get()?

hard

A. try:
product = Product.objects.get(sku='12345')
except Product.DoesNotExist:
product = Product.objects.create(sku='12345', name='New Product')

B. product = Product.objects.get_or_create(sku='12345', name='New Product')

C. product = Product.objects.get(sku='12345') or Product.objects.create(sku='12345', name='New Product')

D. product = Product.objects.filter(sku='12345').get_or_create(name='New Product')

Solution

1. Step 1: Understand get() behavior and exception handling

   get() raises DoesNotExist if no object matches, so use try-except to handle this.

2. Step 2: Review options for correct usage

   try: product = Product.objects.get(sku='12345') except Product.DoesNotExist: product = Product.objects.create(sku='12345', name='New Product') uses try-except with get() and creates the object if not found, which is correct. product = Product.objects.get_or_create(sku='12345', name='New Product') uses get_or_create() which is a different method, not get(). The other options misuse method chaining and will cause errors.

3. Final Answer:

   Use try-except with get() and create if not found. -> Option A

4. Quick Check:

   Handle DoesNotExist with try-except for get() [OK]

Hint: Use try-except DoesNotExist to handle get() missing object [OK]

Common Mistakes:

• Using get_or_create() instead of get()
• Assuming get() returns None if no match
• Chaining get_or_create() after filter() incorrectly