Select and pluck in Ruby on Rails - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all users with age greater than 20.

Ruby on Rails

users = User.where("age [1] 20")

Drag options to blanks, or click blank then click option'

A>

B<

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' selects younger users.

Using '=' selects only age 20.

The where method filters records. Using age > 20 selects users older than 20.

2fill in blank

medium

Complete the code to pluck the names of all users.

Ruby on Rails

names = User.[1](:name)

Drag options to blanks, or click blank then click option'

Aselect

Bpluck

Cwhere

Dorder

Attempts:

3 left

💡 Hint

Common Mistakes

Using select returns full objects, not just values.

Using where filters records but doesn't extract values.

The pluck method extracts a single column's values directly from the database.

3fill in blank

hard

Fix the error in the code to select users with name 'Alice'.

Ruby on Rails

users = User.where(name: [1])

Drag options to blanks, or click blank then click option'

AAlice

B:Alice

C'Alice'

D"Alice"

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted words causes errors.

Using symbols instead of strings for string values.

In Ruby, string values in hashes should be quoted with single or double quotes. Here, 'Alice' is correct.

4fill in blank

hard

Fill both blanks to select users older than 30 and pluck their emails.

Ruby on Rails

emails = User.where("age [1] 30").[2](:email)

Drag options to blanks, or click blank then click option'

A>

Bpluck

Cselect

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' selects younger users.

Using select returns full objects, not just emails.

Use where("age > 30") to filter users older than 30, then pluck(:email) to get their emails.

5fill in blank

hard

Fill all three blanks to select users with age less than 25, order by name, and pluck their ids.

Ruby on Rails

ids = User.where("age [1] 25").[2](:name).[3](:id)

Drag options to blanks, or click blank then click option'

A<

Border

Cpluck

Dwhere

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operator.

Using where instead of order for sorting.

Using select instead of pluck for extracting ids.

Filter users with age < 25, order them by name, then pluck their id values.

Practice

1. What is the main difference between select and pluck in Rails ActiveRecord?

easy

A. select returns all columns, pluck returns only the first column.

B. select returns model objects with selected columns, pluck returns an array of values.

C. select modifies the database, pluck deletes records.

D. select returns an array of values, pluck returns model objects.

Solution

1. Step 1: Understand select behavior

   select loads model objects but only with the specified columns, so you still get ActiveRecord objects.

2. Step 2: Understand pluck behavior

   pluck fetches column values directly from the database and returns them as a simple array, not objects.

3. Final Answer:

   select returns model objects with selected columns, pluck returns an array of values. -> Option B

4. Quick Check:

   Select = objects, Pluck = array [OK]

Hint: Select returns objects, pluck returns arrays of values [OK]

Common Mistakes:

• Thinking pluck returns objects
• Confusing select with all columns
• Assuming select modifies data

2. Which of the following is the correct syntax to get an array of user emails using pluck?

easy

A. User.pluck(:email)

B. User.select(:email).pluck

C. User.pluck.email

D. User.select.pluck(:email)

Solution

1. Step 1: Identify correct method usage

   pluck is called on the model with the column symbol as argument, like User.pluck(:email).

2. Step 2: Check other options for syntax errors

   Options A, B, and C misuse method chaining or syntax and will cause errors.

3. Final Answer:

   User.pluck(:email) -> Option A

4. Quick Check:

   Correct pluck syntax = User.pluck(:column) [OK]

Hint: Use Model.pluck(:column) to get array of values [OK]

Common Mistakes:

• Calling pluck without parentheses
• Trying to chain pluck after select incorrectly
• Using dot notation like pluck.email

3. Given the following code, what will be the output?

users = User.select(:id, :name).where(active: true)
names = users.pluck(:name)

medium

A. An error because pluck cannot be called after select

B. An array of all user names regardless of active status

C. An array of names of active users

D. An array of User objects with id and name

Solution

1. Step 1: Analyze the query chain

   User.select(:id, :name).where(active: true) returns ActiveRecord objects with only id and name for active users.

2. Step 2: Understand pluck on the relation

   Calling pluck(:name) on this relation fetches the name column values directly from the database for active users.

3. Final Answer:

   An array of names of active users -> Option C

4. Quick Check:

   Pluck after where filters data correctly [OK]

Hint: Pluck after where returns filtered column values [OK]

Common Mistakes:

• Thinking pluck returns objects
• Assuming pluck ignores where filters
• Believing pluck cannot follow select

4. Identify the error in this code snippet:

users = User.pluck(:id, :name).select { |u| u.active? }

medium

A. No error, code works fine

B. Pluck cannot take multiple columns

C. Select cannot be called after pluck

D. Calling active? on array elements causes error

Solution

1. Step 1: Understand what pluck returns

   pluck(:id, :name) returns an array of arrays like [[1, "Alice"], [2, "Bob"]], not User objects.

2. Step 2: Check usage of active?

   Since elements are arrays, calling active? method on them causes a NoMethodError.

3. Final Answer:

   Calling active? on array elements causes error -> Option D

4. Quick Check:

   Pluck returns arrays, not objects [OK]

Hint: Pluck returns arrays, not objects with methods [OK]

Common Mistakes:

• Assuming pluck returns objects
• Trying to call model methods on pluck results
• Thinking select after pluck filters objects

5. You want to get a list of unique active user emails efficiently. Which code snippet is best?

hard

A. User.where(active: true).pluck(:email).uniq

B. User.where(active: true).select(:email).map(&:email).uniq

C. User.select(:email).where(active: true).pluck(:email).uniq

D. User.pluck(:email).where(active: true).uniq

Solution

1. Step 1: Analyze efficiency of each option

   User.where(active: true).select(:email).map(&:email).uniq loads objects with select then maps emails, which loads more data than needed.

2. Step 2: Check correct chaining and method usage

   User.where(active: true).pluck(:email).uniq uses where to filter active users, then pluck to get emails directly from DB, then uniq to remove duplicates efficiently.

3. Step 3: Identify errors in other options

   User.select(:email).where(active: true).pluck(:email).uniq is redundant but works; User.pluck(:email).where(active: true).uniq is invalid because pluck returns array, so where after pluck causes error.

4. Final Answer:

   User.where(active: true).pluck(:email).uniq -> Option A

5. Quick Check:

   Filter then pluck for efficiency [OK]

Hint: Filter with where, then pluck columns, then uniq [OK]

Common Mistakes:

• Calling where after pluck (invalid)
• Loading objects unnecessarily before pluck
• Not removing duplicates efficiently