limit method for pagination in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What documents are returned by this MongoDB query?

Consider a collection books with documents sorted by publishedYear ascending. What documents will this query return?

db.books.find().sort({publishedYear: 1}).limit(3)

MongoDB

db.books.find().sort({publishedYear: 1}).limit(3)

A The first 3 books with the earliest publishedYear values.

B The last 3 books with the latest publishedYear values.

C All books sorted by publishedYear, but only those with publishedYear less than 3.

D 3 random books from the collection.

💡 Hint

Remember, limit(n) returns only the first n documents after sorting.

The query sorts all books by publishedYear ascending (earliest first), then limits the output to the first 3 documents in that order.

❓ query_result

intermediate

What is the output of this MongoDB query with skip and limit?

Given a collection users sorted by age ascending, what documents does this query return?

db.users.find().sort({age: 1}).skip(5).limit(4)

MongoDB

db.users.find().sort({age: 1}).skip(5).limit(4)

A Users ranked 1st to 4th youngest by age.

B Users ranked 6th to 9th youngest by age.

C Users ranked 5th to 8th youngest by age.

D Users ranked 5th to 9th youngest by age.

💡 Hint

Skip removes the first 5 documents, then limit returns the next 4.

The query skips the first 5 users (youngest 5), then returns the next 4 users sorted by age ascending, which are users ranked 6th to 9th youngest.

📝 Syntax

advanced

Which MongoDB query correctly limits results to 10 documents after sorting by score descending?

Choose the query that correctly returns the top 10 documents sorted by score descending.

A db.collection.find().limit(10).sort({score: -1})

B db.collection.find().sort({score: 1}).limit(10)

C db.collection.find().limit(10).sort({score: 1})

D db.collection.find().sort({score: -1}).limit(10)

💡 Hint

Sorting must happen before limiting to get the top scores.

The correct order is to sort first by score descending, then limit to 10 documents. Option D does this correctly.

❓ optimization

advanced

How to optimize pagination for large collections using limit and skip?

You want to paginate through a large MongoDB collection efficiently. Which approach is best to avoid performance issues with skip and limit?

A Use <code>limit</code> only without <code>skip</code> to get the first page repeatedly.

B Use <code>skip</code> with <code>limit</code> but add <code>sort</code> on a non-indexed field.

C Use a range query on an indexed field (like _id) with <code>limit</code> instead of large skips.

D Use <code>skip</code> with large offsets and <code>limit</code> as usual.

💡 Hint

Large skips cause MongoDB to scan many documents internally.

Using range queries on indexed fields avoids scanning skipped documents, improving pagination performance.

🧠 Conceptual

expert

Why does using only limit without sort cause inconsistent pagination results?

In MongoDB, if you paginate using only limit without specifying sort, what problem can occur?

A Documents may appear in different orders on different queries, causing inconsistent pages.

B The query will always return the same documents regardless of changes in the collection.

C MongoDB will throw a syntax error because <code>limit</code> requires <code>sort</code>.

D The query will return documents sorted by _id automatically, so pagination is stable.

💡 Hint

Without sorting, MongoDB does not guarantee order of documents returned.

Without a sort, the order of documents returned is not guaranteed and can change, leading to inconsistent pagination results.

Practice

1. What does the limit() method do in MongoDB queries?

easy

A. It updates documents in the collection.

B. It sorts the documents in ascending order.

C. It skips a specified number of documents.

D. It restricts the number of documents returned by the query.

Solution

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

   The limit() method is used to control how many documents a query returns.

2. Step 2: Compare with other methods

   Unlike sort() which orders documents, or skip() which skips documents, limit() restricts the count of results.

3. Final Answer:

   It restricts the number of documents returned by the query. -> Option D

4. Quick Check:

   limit() controls result count = D [OK]

Hint: Limit sets max results returned, like page size [OK]

Common Mistakes:

• Confusing limit with skip
• Thinking limit sorts results
• Assuming limit updates data

2. Which of the following is the correct syntax to limit a MongoDB query to 5 documents?

easy

A. db.collection.find().limit(5)

B. db.collection.limit(5).find()

C. db.collection.find().skip(5)

D. db.collection.find(5).limit()

Solution

1. Step 1: Recall the correct method chaining

   In MongoDB, limit() is chained after find() to restrict results.

2. Step 2: Validate syntax correctness

   db.collection.find().limit(5) uses db.collection.find().limit(5), which is the correct syntax. Other options misuse method order or parameters.

3. Final Answer:

   db.collection.find().limit(5) -> Option A

4. Quick Check:

   Correct chaining is find() then limit() = A [OK]

Hint: Use find() first, then limit(n) to restrict results [OK]

Common Mistakes:

• Placing limit before find
• Using skip instead of limit
• Passing number inside find()

3. Given the collection products with documents: [{"name":"A"},{"name":"B"},{"name":"C"},{"name":"D"}], what will db.products.find().limit(2).toArray() return?

medium

A. [{"name":"A"},{"name":"B"},{"name":"C"}]

B. [{"name":"C"},{"name":"D"}]

C. [{"name":"A"},{"name":"B"}]

D. An error because limit cannot be used here

Solution

1. Step 1: Understand the default order of find()

   Without sorting, find() returns documents in insertion order: A, B, C, D.

2. Step 2: Apply limit(2) to restrict results

   Using limit(2) returns only the first two documents: A and B.

3. Final Answer:

   [{"name":"A"},{"name":"B"}] -> Option C

4. Quick Check:

   limit(2) returns first 2 docs = A [OK]

Hint: Limit returns first N documents in default order [OK]

Common Mistakes:

• Assuming limit returns last documents
• Expecting sorted results without sort()
• Thinking limit causes errors

4. What is wrong with this query for pagination?

db.users.find().limit(10).skip(5)

medium

A. The query is missing a sort() method for consistent pagination.

B. The order of limit and skip is incorrect; it causes a syntax error.

C. limit cannot be used with skip in the same query.

D. There is no error; the query is correct.

Solution

1. Step 1: Check method order validity

   In MongoDB, chaining limit() and skip() in any order is syntactically valid.

2. Step 2: Identify pagination best practice

   For consistent pagination, a sort() is needed to ensure stable document order across pages.

3. Final Answer:

   The query is missing a sort() method for consistent pagination. -> Option A

4. Quick Check:

   Pagination needs sort() for stable results = B [OK]

Hint: Always add sort() for reliable pagination [OK]

Common Mistakes:

• Thinking the order causes a syntax error
• Believing limit and skip can't be combined
• Ignoring the need for sort()

5. You want to show page 3 of a product list with 4 items per page, sorted by price ascending. Which query correctly uses limit() and skip() for this pagination?

hard

A. db.products.find().skip(4).limit(3).sort({price:1})

B. db.products.find().sort({price:1}).skip(8).limit(4)

C. db.products.find().limit(4).skip(8).sort({price:1})

D. db.products.find().sort({price:1}).limit(3).skip(4)

Solution

1. Step 1: Calculate skip value for page 3

   Each page has 4 items, so page 3 skips 2 pages: 2 * 4 = 8 documents to skip.

2. Step 2: Apply sort, skip, and limit in correct order

   Sort by price ascending first, then skip 8 documents, then limit to 4 documents for the page size.

3. Final Answer:

   db.products.find().sort({price:1}).skip(8).limit(4) -> Option B

4. Quick Check:

   Page 3 skip=8, limit=4, sorted = C [OK]

Hint: Skip (page-1)*size, then limit size after sort [OK]

Common Mistakes:

• Mixing skip and limit order
• Wrong skip calculation for page
• Missing sort for consistent order