Query interface basics in Ruby on Rails - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to fetch all users from the database.

Ruby on Rails

users = User.[1]

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

Aselect

Bfind

Cwhere

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'find' without an ID causes an error.

Using 'where' requires conditions.

Using 'select' is for choosing columns, not records.

The all method fetches all records from the users table.

2fill in blank

medium

Complete the code to find users with the name 'Alice'.

Ruby on Rails

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

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

A'Alice'

B:Alice

CAlice

D"Alice"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a symbol (:Alice) instead of a string.

Not quoting the string at all.

Using bare words without quotes.

In Rails queries, string values must be quoted with single or double quotes. Here, 'Alice' is correct.

3fill in blank

hard

Fix the error in the code to get the first user ordered by created_at.

Ruby on Rails

user = User.order(:created_at).[1]

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

Alast

Bfirst

Ctake

Dfind

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'last' returns the newest record, not the first.

Using 'find' requires an ID argument.

Using 'take' returns an array, while 'first' returns a single record.

The first method returns the first record in the ordered list.

4fill in blank

hard

Fill both blanks to get users created after 2023-01-01 and order by name.

Ruby on Rails

users = User.where("created_at > [1]").[2](:name)

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

A'2023-01-01'

Border

Csort

Dwhere

Attempts:

3 left

💡 Hint

Common Mistakes

Using sort instead of order.

Not quoting the date string.

Using where again instead of order.

The date must be a string in quotes. The order method sorts the results by the given column.

5fill in blank

hard

Fill all three blanks to select users with age over 30, order by age descending, and limit to 5 results.

Ruby on Rails

users = User.where('age [1] [2]').order(age: [3]).limit(5)

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

A>

B30

C:desc

D:asc

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' for age.

Ordering ascending instead of descending.

Quoting the number 30 in the where clause.

The query filters users older than 30 using '>'. The order is descending with :desc.

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