Order and limit in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this ActiveRecord query?

Given a Rails model Product with a price attribute, what will this query return?

Product.order(price: :desc).limit(3).pluck(:price)

A An array of all product prices sorted descending, ignoring the limit.

B An array of the three lowest prices from the products table, sorted ascending.

C An array of the three highest prices from the products table, sorted descending.

D An array of three random product prices without any specific order.

💡 Hint

Remember that order sorts the results and limit restricts how many records are returned.

The query orders products by price descending, then limits the result to 3 records, and finally plucks the price values. So it returns the top 3 highest prices.

📝 Syntax

intermediate

Which option correctly orders users by creation date ascending and limits to 5?

Choose the valid Rails ActiveRecord query that orders users by created_at ascending and limits the result to 5 records.

A User.order('created_at asc').limit('5')

B User.limit(5).order(created_at: :asc)

C User.order(created_at: asc).limit(5)

D User.order(:created_at).limit(5)

💡 Hint

Check the syntax for specifying order direction and limit argument types.

Option D uses the correct symbol for ordering ascending (:created_at implies ASC) and passes an integer to limit. Option D has a Ruby syntax error (`created_at` undefined). Option D has syntax errors (`created_at` and `asc` undefined). Option D passes limit as a string which is invalid.

🔧 Debug

advanced

Why does this query raise an error?

Consider this Rails query:

Post.order('title DESC').limit('10')

Why does it raise an error?

A Because order cannot accept a string argument.

B Because the limit argument must be an integer, not a string.

C Because limit must come before order in the query chain.

D Because 'title DESC' is not a valid column name.

💡 Hint

Check the data types expected by limit.

The limit method expects an integer argument. Passing a string like '10' causes a type error.

❓ state_output

advanced

What is the number of records returned by this query?

Given a Comment model with 15 records, what is the count of records returned by this query?

Comment.order(created_at: :desc).limit(20)

A 15

B 20

C Raises an error because limit is greater than total records.

D 0

💡 Hint

Limit restricts maximum records but does not add records if fewer exist.

The query limits to 20 records but only 15 exist, so all 15 are returned.

🧠 Conceptual

expert

Which option best explains the difference between limit and take in Rails?

Select the correct explanation about how limit and take behave differently in ActiveRecord queries.

A <code>limit(n)</code> restricts the number of records returned, while <code>take(n)</code> fetches n records without any specific order, often using SQL LIMIT.

B <code>limit(n)</code> always orders records by primary key ascending, <code>take(n)</code> orders by created_at descending.

C <code>limit(n)</code> fetches records lazily, <code>take(n)</code> fetches all records and then slices in Ruby.

D <code>limit(n)</code> returns an array, <code>take(n)</code> returns an ActiveRecord relation.

💡 Hint

Think about how these methods translate to SQL and their default ordering.

limit(n) adds a SQL LIMIT clause to restrict results. take(n) fetches n records but does not guarantee order unless specified. take is often used to get a fixed number of records quickly without ordering.

Practice

1. What does the order method do in Rails ActiveRecord queries?

easy

A. Sorts the records by a specified column

B. Limits the number of records returned

C. Filters records based on conditions

D. Joins two tables together

Solution

1. Step 1: Understand the purpose of order

   The order method sorts records by the column you specify, like sorting a list alphabetically.

2. Step 2: Differentiate from other methods

   limit controls how many records you get, where filters records, and joins combines tables. Only order sorts.

3. Final Answer:

   Sorts the records by a specified column -> Option A

4. Quick Check:

   Order = Sort [OK]

Hint: Order sorts data by column, limit controls count [OK]

Common Mistakes:

• Confusing order with limit
• Thinking order filters records
• Mixing order with joins

2. Which of the following is the correct syntax to get the first 5 users ordered by their creation date in Rails?

easy

A. User.limit(5).order(created_at)

B. User.order(:created_at).limit(5)

C. User.order_by(:created_at).limit(5)

D. User.limit(5).sort(:created_at)

Solution

1. Step 1: Check method names and order

   The correct ActiveRecord methods are order and limit. The method order_by and sort do not exist in this context.

2. Step 2: Confirm chaining order

   You can chain order then limit or vice versa, but the common and recommended way is order(:created_at).limit(5).

3. Final Answer:

   User.order(:created_at).limit(5) -> Option B

4. Quick Check:

   Correct syntax = User.order(:created_at).limit(5) [OK]

Hint: Use order then limit with correct method names [OK]

Common Mistakes:

• Using non-existent methods like order_by or sort
• Swapping method names incorrectly
• Incorrect chaining order causing errors

3. Given the code Product.order(price: :desc).limit(3).pluck(:price), what will be the output if prices are [10, 50, 30, 20, 40]?

medium

A. [10, 20, 30]

B. [40, 30, 20]

C. [50, 40, 30]

D. [30, 20, 10]

Solution

1. Step 1: Understand ordering by price descending

   The prices sorted descending are [50, 40, 30, 20, 10].

2. Step 2: Apply limit and pluck

   Limiting to 3 means we take the first three prices: [50, 40, 30].

3. Final Answer:

   [50, 40, 30] -> Option C

4. Quick Check:

   Order desc + limit 3 = [50, 40, 30] [OK]

Hint: Order desc sorts highest first, limit picks top records [OK]

Common Mistakes:

• Forgetting descending order
• Mixing ascending with descending
• Ignoring limit count

4. What is wrong with this Rails query? User.limit(5).order('name')

medium

A. limit cannot be used with order

B. The order argument should be a symbol, not a string

C. The order method must come before limit

D. The query is valid and will work correctly

Solution

1. Step 1: Check method chaining order

   In Rails, chaining limit before or after order works fine; order does not have to come first.

2. Step 2: Check argument type for order

   Passing a string like 'name' to order is valid syntax in Rails.

3. Final Answer:

   The query is valid and will work correctly -> Option D

4. Quick Check:

   limit + order chaining is flexible = The query is valid and will work correctly [OK]

Hint: Order and limit can be chained in any order [OK]

Common Mistakes:

• Thinking order must come before limit
• Believing order needs symbol not string
• Assuming limit and order cannot combine

5. You want to get the 3 most recent posts but only those with status 'published'. Which Rails query correctly combines filtering, ordering by newest first, and limiting?

hard

A. Post.where(status: 'published').order(created_at: :desc).limit(3)

B. Post.limit(3).order(created_at: :asc).where(status: 'published')

C. Post.order(:created_at).where(status: 'published').limit(3)

D. Post.where(status: 'published').limit(3).order(created_at: :asc)

Solution

1. Step 1: Filter posts by status 'published'

   Use where(status: 'published') to get only published posts.

2. Step 2: Order posts by newest first

   Use order(created_at: :desc) to sort posts from newest to oldest.

3. Step 3: Limit to 3 records

   Use limit(3) to get only the top 3 posts after filtering and ordering.

4. Final Answer:

   Post.where(status: 'published').order(created_at: :desc).limit(3) -> Option A

5. Quick Check:

   Filter + order desc + limit = Post.where(status: 'published').order(created_at: :desc).limit(3) [OK]

Hint: Filter first, then order desc, then limit [OK]

Common Mistakes:

• Limiting before filtering causing wrong results
• Ordering ascending instead of descending
• Mixing method order causing unexpected output