get() for single objects in Django - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the get() method do in Django's QuerySet?

The get() method retrieves a single object from the database that matches the given filter criteria. If no object or more than one object matches, it raises an error.

beginner

What exception does get() raise if no object matches the query?

It raises a DoesNotExist exception specific to the model class.

beginner

What happens if get() finds more than one object matching the query?

It raises a MultipleObjectsReturned exception to indicate the query was not specific enough.

beginner

How do you use get() to find a user with username 'alice'?

You write: User.objects.get(username='alice'). This returns the user object with username 'alice' or raises an error if not found or multiple found.

intermediate

Why is get() preferred over filter() when you expect only one object?

get() returns a single object directly and raises errors if the result is not exactly one, helping catch mistakes early. filter() returns a list-like QuerySet even if one or zero objects match.

What does Model.objects.get(id=1) return if exactly one object with id=1 exists?

A Raises MultipleObjectsReturned

B A list of objects with <code>id=1</code>

C An empty QuerySet

D The single object with <code>id=1</code>

get() returns the single matching object if exactly one exists.

What exception is raised by get() if no object matches the query?

A DoesNotExist

B MultipleObjectsReturned

C ValueError

D KeyError

get() raises DoesNotExist if no matching object is found.

If get() finds two objects matching the query, what happens?

A Returns the first object

B Returns a list of objects

C Raises MultipleObjectsReturned

D Returns None

get() raises MultipleObjectsReturned if more than one object matches.

Which method should you use to get exactly one object or fail with an error?

A get()

B exclude()

C all()

D filter()

get() is designed to return exactly one object or raise an error.

What is the return type of get()?

A A list

B A single model instance

C A QuerySet

D A dictionary

get() returns a single model instance, not a QuerySet or list.

Explain how Django's get() method works for retrieving single objects.

Think about what happens if zero or multiple objects match.

Describe the difference between get() and filter() in Django QuerySets.

Consider the return types and error handling.

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