Ordering and slicing querysets in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this queryset slicing?

Given a Django model Book with 10 entries ordered by published_date, what does Book.objects.order_by('published_date')[2:5] return?

A A queryset with the 2nd, 3rd, and 4th books ordered by published_date

B A queryset with the last 3 books ordered by published_date

C A queryset with the 3rd, 4th, and 5th books ordered by published_date

D A queryset with the 1st, 2nd, and 3rd books ordered by published_date

💡 Hint

Remember Python slicing starts at index 0 and excludes the stop index.

The slice [2:5] selects items starting at index 2 up to but not including index 5. So it returns the 3rd, 4th, and 5th books in the ordered queryset.

📝 Syntax

intermediate

Which queryset ordering syntax is correct?

Select the correct way to order a queryset of Author by last name descending and then first name ascending.

A Author.objects.order_by('last_name', '-first_name')

B Author.objects.order_by('-last_name', 'first_name')

C Author.objects.order_by('last_name-', 'first_name')

D Author.objects.order_by('last_name desc', 'first_name asc')

💡 Hint

Use a minus sign before the field name for descending order.

In Django, prefixing a field name with - orders descending. So order_by('-last_name', 'first_name') orders by last name descending, then first name ascending.

🔧 Debug

advanced

Why does this queryset slicing cause an error?

Consider this code: books = Book.objects.order_by('title')[5:2]. What happens when this runs?

A Returns an empty queryset because the slice start is greater than stop

B Raises a SyntaxError due to invalid slice syntax

C Returns the last 3 books ordered by title

D Raises a ValueError because slicing with start > stop is not allowed

💡 Hint

Think about how Python slicing behaves when start index is larger than stop index.

Python slicing with start index greater than stop index returns an empty list or queryset. It does not raise an error.

🧠 Conceptual

advanced

How does chaining order_by calls affect the queryset?

What is the result of this queryset: Book.objects.order_by('author').order_by('published_date')?

A The queryset is ordered only by published_date, ignoring author

B The queryset is ordered by author and published_date combined

C The queryset is ordered first by author, then by published_date

D Raises an error because multiple order_by calls are not allowed

💡 Hint

Each order_by call overrides the previous ordering.

In Django, calling order_by multiple times replaces the previous ordering. Only the last order_by call applies.

❓ state_output

expert

What is the number of items in this sliced queryset?

If Entry.objects.all() returns 15 items, how many items does Entry.objects.order_by('-created_at')[3:10] contain?

A 12 items

B 6 items

C 10 items

D 7 items

💡 Hint

Count the items between indices 3 and 9 inclusive.

The slice [3:10] includes items at indices 3 through 9, which is 7 items total.

Practice

1. What does the Django queryset method order_by('name') do?

easy

A. Groups the queryset results by the 'name' field.

B. Filters the queryset to only include objects with the name 'order_by'.

C. Sorts the queryset results by the 'name' field in ascending order.

D. Deletes all objects that have a 'name' field.

Solution

1. Step 1: Understand the order_by() method

   The order_by() method sorts the queryset results based on the given field(s).

2. Step 2: Apply order_by('name')

   This sorts the results by the 'name' field in ascending order by default.

3. Final Answer:

   Sorts the queryset results by the 'name' field in ascending order. -> Option C

4. Quick Check:

   order_by('name') = sorted by name ascending [OK]

Hint: Remember: order_by sorts, does not filter or delete [OK]

Common Mistakes:

• Confusing order_by with filter
• Thinking order_by deletes data
• Assuming order_by groups data

2. Which of the following is the correct syntax to get the first 5 objects ordered by 'created_at' descending?

easy

A. Model.objects.order_by('-created_at')[:5]

B. Model.objects[:5].order_by('-created_at')

C. Model.objects.order_by('created_at')[:5]

D. Model.objects.order_by('created_at')[-5:]

Solution

1. Step 1: Use order_by('-created_at') for descending order

   Prefixing the field with '-' sorts descending.

2. Step 2: Slice the queryset with [:5] to get first 5 results

   Slicing before evaluation limits results to first 5.

3. Final Answer:

   Model.objects.order_by('-created_at')[:5] -> Option A

4. Quick Check:

   Descending order + first 5 = order_by('-field')[:5] [OK]

Hint: Use '-' before field for descending order, slice after ordering [OK]

Common Mistakes:

• Slicing before ordering (wrong order)
• Missing '-' for descending order
• Using negative slice like [-5:] incorrectly

3. Given the queryset qs = Model.objects.order_by('age')[2:5], what will list(qs) return if the ages in the database are [20, 25, 30, 35, 40, 45]?

medium

A. [25, 30, 35]

B. [30, 35, 40]

C. [35, 40, 45]

D. [20, 25, 30]

Solution

1. Step 1: Order the ages ascending

   Ordering by 'age' gives [20, 25, 30, 35, 40, 45].

2. Step 2: Slice from index 2 to 5 (excluding 5)

   Indexes 2, 3, 4 correspond to ages 30, 35, 40.

3. Final Answer:

   [30, 35, 40] -> Option B

4. Quick Check:

   order_by + slice = [30, 35, 40] [OK]

Hint: Remember slicing excludes the end index [OK]

Common Mistakes:

• Including the end index in slice
• Mixing up ascending and descending order
• Using wrong slice indexes

4. What is wrong with this queryset code?

qs = Model.objects[:5].order_by('name')

medium

A. You cannot slice a queryset before ordering; slicing must come after ordering.

B. The queryset must be filtered before ordering.

C. The order_by method requires a list, not a string.

D. Slicing with [:5] returns a list, so order_by cannot be called after.

Solution

1. Step 1: Understand queryset slicing returns a list

   Slicing a queryset like Model.objects[:5] evaluates it and returns a list, not a queryset.

2. Step 2: Calling order_by on a list causes error

   Lists do not have order_by method, so this code raises an error.

3. Final Answer:

   Slicing with [:5] returns a list, so order_by cannot be called after. -> Option D

4. Quick Check:

   Slice first = list, no order_by after [OK]

Hint: Always order before slicing to keep queryset chainable [OK]

Common Mistakes:

• Slicing before ordering
• Thinking order_by accepts lists
• Confusing filter and order_by order

5. You want to get the 3rd to 7th newest entries from a model ordered by 'published_date' descending. Which queryset code is correct?

hard

A. Model.objects.order_by('-published_date')[2:7]

B. Model.objects.order_by('published_date')[3:8]

C. Model.objects[2:7].order_by('-published_date')

D. Model.objects.order_by('-published_date')[3:8]

Solution

1. Step 1: Order by 'published_date' descending

   Use order_by('-published_date') to get newest first.

2. Step 2: Slice from index 2 to 7 to get 3rd to 7th entries

   Slicing [2:7] gets items at indexes 2,3,4,5,6 (5 items total).

3. Final Answer:

   Model.objects.order_by('-published_date')[2:7] -> Option A

4. Quick Check:

   Descending order + slice 2:7 = 3rd to 7th newest [OK]

Hint: Order descending first, then slice with zero-based indexes [OK]

Common Mistakes:

• Using ascending order instead of descending
• Slicing with wrong indexes (off by one)
• Slicing before ordering