Query interface basics 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 User with records having age attributes, what does this query return?

User.where(age: 20..30).pluck(:name)

A An array of user ages between 20 and 30

B An array of all user names regardless of age

C An array of names of users whose age is between 20 and 30 inclusive

D A hash with user names as keys and ages as values for users aged 20 to 30

💡 Hint

Think about what where and pluck do in ActiveRecord.

The where(age: 20..30) filters users with age between 20 and 30 inclusive. The pluck(:name) extracts only the name attribute values from those filtered records, returning an array of names.

📝 Syntax

intermediate

Which option correctly finds users created after 2020-01-01?

Select the correct ActiveRecord query to find users created after January 1, 2020.

A User.where('created_at > ?', '2020-01-01')

B User.where(created_at: '> 2020-01-01')

C User.where(created_at > '2020-01-01')

D User.where(created_at: ['2020-01-01'..])

💡 Hint

Remember how to use raw SQL conditions with placeholders in where.

Option A uses a raw SQL condition with a placeholder, which is the correct syntax. Option A is invalid syntax for range or comparison. Option A is invalid Ruby syntax. Option A uses an invalid range syntax in a hash.

❓ state_output

advanced

What is the count of users returned by this query?

Assuming the users table has 10 records with active true and 5 with false, what does this return?

User.where(active: true).limit(3).count

A 5

B 10

C 15

D 3

💡 Hint

Remember how count works with limit in ActiveRecord.

The count method respects limit and counts only the limited number of records. So it counts 3 active users, not all 10.

🔧 Debug

advanced

Why does this query raise an error?

Given the query:

User.where(name: 'Alice').order(:nonexistent_column)

Why does it raise an error?

A Because <code>nonexistent_column</code> does not exist in the users table

B Because <code>name</code> is not a valid attribute

C Because <code>order</code> cannot be used after <code>where</code>

D Because <code>where</code> requires a block

💡 Hint

Check the column names in the database schema.

The error occurs because order tries to sort by a column that does not exist in the database table, causing a database error.

🧠 Conceptual

expert

Which option best describes lazy loading in ActiveRecord queries?

Select the statement that correctly explains lazy loading behavior in ActiveRecord query interface.

A Queries are executed in the background asynchronously by default

B Queries are executed only when the data is actually needed, like when iterating or accessing records

C Queries are never executed automatically and must be manually triggered

D Queries are executed immediately when the query method is called

💡 Hint

Think about when ActiveRecord fetches data from the database.

ActiveRecord builds query objects lazily. The actual database query runs only when the data is accessed, such as by iterating or calling methods like to_a.

Practice

1. What does the where method do in Rails' query interface?

easy

A. Filters records based on given conditions

B. Deletes records from the database

C. Updates records with new values

D. Creates new records in the database

Solution

1. Step 1: Understand the purpose of where

   The where method is used to filter records by specifying conditions, like filtering users by age.

2. Step 2: Compare with other methods

   Methods like delete, update, and create perform different actions, not filtering.

3. Final Answer:

   Filters records based on given conditions -> Option A

4. Quick Check:

   where filters records [OK]

Hint: Remember: where = filter records by condition [OK]

Common Mistakes:

• Confusing where with update or delete
• Thinking where creates or deletes records
• Assuming where changes data instead of filtering

2. Which of the following is the correct syntax to get all users ordered by their created_at date in descending order?

easy

A. User.order(:created_at, :desc)

B. User.order('created_at ASC')

C. User.order(created_at: :desc)

D. User.order(created_at => 'desc')

Solution

1. Step 1: Identify correct hash syntax for order

   Rails expects a hash with column name as key and direction as symbol, like created_at: :desc.

2. Step 2: Check other options for syntax errors

   User.order(:created_at, :desc) uses wrong argument format treating :desc as column name, User.order('created_at ASC') uses string with wrong direction, User.order(created_at => 'desc') uses wrong hash rocket syntax.

3. Final Answer:

   User.order(created_at: :desc) -> Option C

4. Quick Check:

   Correct hash syntax for order = User.order(created_at: :desc) [OK]

Hint: Use symbol keys and symbols for direction in order [OK]

Common Mistakes:

• Passing multiple arguments instead of a hash
• Using wrong direction like ASC in order string
• Wrong hash syntax with => and string

3. Given the code Product.where(price: 100).limit(3).pluck(:name), what will be the output?

medium

A. An array of up to 3 product names with price 100

B. An array of all product names regardless of price

C. A single product name with price 100

D. An error because limit cannot be chained after where

Solution

1. Step 1: Understand chaining of query methods

   where(price: 100) filters products with price 100, limit(3) restricts to 3 records, pluck(:name) extracts names into an array.

2. Step 2: Analyze output type

   The result is an array of names, up to 3, matching the price condition.

3. Final Answer:

   An array of up to 3 product names with price 100 -> Option A

4. Quick Check:

   where + limit + pluck = filtered limited array [OK]

Hint: Chain filters, limits, then pluck for array output [OK]

Common Mistakes:

• Thinking pluck returns a single value
• Assuming limit cannot be chained after where
• Ignoring the filter condition in output

4. What is wrong with this query? User.where('age > ?', 18).order('name ASC').limit('five')

medium

A. The where clause syntax is incorrect

B. The limit argument should be an integer, not a string

C. The order method cannot use strings

D. The query is correct and will run without errors

Solution

1. Step 1: Check each method argument type

   where('age > ?', 18) is correct, order('name ASC') accepts string, but limit('five') expects an integer, not a string.

2. Step 2: Identify the error source

   Passing 'five' as a string sets limit to 0 because 'five'.to_i == 0, not the intended value.

3. Final Answer:

   The limit argument should be an integer, not a string -> Option B

4. Quick Check:

   limit needs integer, not string [OK]

Hint: limit must be a number, not a word string [OK]

Common Mistakes:

• Passing string instead of integer to limit
• Thinking order cannot take strings
• Misreading where clause syntax

5. You want to find all Article records published after 2020, sorted by title ascending, but only show the first 5 titles. Which query is correct?

hard

A. Article.where(published_at: > Date.new(2020,1,1)).order(:title).limit(5).pluck(:title)

B. Article.limit(5).order(title: :asc).where('published_at > 2020-01-01').pluck(:title)

C. Article.where('published_at > 2020-01-01').order('title DESC').limit(5).pluck(:title)

D. Article.where('published_at > ?', Date.new(2020,1,1)).order(title: :asc).limit(5).pluck(:title)

Solution

1. Step 1: Check filtering condition syntax

   Article.where('published_at > ?', Date.new(2020,1,1)).order(title: :asc).limit(5).pluck(:title) uses correct SQL placeholder with where('published_at > ?', Date.new(2020,1,1)). Article.where(published_at: > Date.new(2020,1,1)).order(:title).limit(5).pluck(:title) has invalid Ruby syntax. Article.where('published_at > 2020-01-01').order('title DESC').limit(5).pluck(:title) uses string date without quotes and wrong order direction.

2. Step 2: Verify order and limit usage

   Article.where('published_at > ?', Date.new(2020,1,1)).order(title: :asc).limit(5).pluck(:title) orders by title ascending with symbol and limits to 5 records before pluck. Article.limit(5).order(title: :asc).where('published_at > ?', '2020-01-01').pluck(:title) applies limit before where, which is less efficient and may give wrong results.

3. Final Answer:

   Article.where('published_at > ?', Date.new(2020,1,1)).order(title: :asc).limit(5).pluck(:title) -> Option D

4. Quick Check:

   Correct where, order ascending, limit 5, pluck titles [OK]

Hint: Filter first, then order, limit, and pluck [OK]

Common Mistakes:

• Wrong where syntax with > operator
• Ordering descending instead of ascending
• Limiting before filtering causing wrong results