Where conditions 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 attributes age and active, what records does this query return?

User.where(age: 30).where(active: true)

A All users who are 30 years old regardless of active status.

B All users who are either 30 years old or active (active = true).

C All users who are exactly 30 years old and active (active = true).

D All users who are active regardless of age.

💡 Hint

Remember that chaining where clauses in Rails combines conditions with AND logic.

Chaining where calls in Rails adds conditions that must all be true. So User.where(age: 30).where(active: true) returns users who are both 30 years old and active.

📝 Syntax

intermediate

Which option correctly uses a hash condition with where?

Select the correct syntax to find all Product records with category equal to 'books' and in_stock equal to true.

A Product.where(category: 'books', in_stock: true)

B Product.where('category = books AND in_stock = true')

C Product.where(category == 'books' && in_stock == true)

D Product.where(category = 'books', in_stock = true)

💡 Hint

Use a Ruby hash with symbols for keys inside where.

The correct syntax uses a hash with keys as symbols and values as the filter values. Option A is valid Ruby and Rails syntax.

🔧 Debug

advanced

Why does this where query raise an error?

Consider this code:

User.where('age > ?', 'twenty')

Why does it raise an error?

A Because 'twenty' is a string and cannot be compared to an integer age.

B Because <code>where</code> does not accept parameterized queries.

C Because the placeholder '?' is missing in the query string.

D Because the column name 'age' is misspelled.

💡 Hint

Check the data type expected for the age column.

The age column expects a number. Passing the string 'twenty' causes a type mismatch error in the database query.

🧠 Conceptual

advanced

What does this where clause do?

What is the effect of this query?

Order.where('created_at >= ?', 1.week.ago).where(status: ['shipped', 'delivered'])

A Returns orders created before one week ago with status 'shipped' or 'delivered'.

B Returns orders created within the last week with status either 'shipped' or 'delivered'.

C Returns orders with status 'shipped' or 'delivered' regardless of creation date.

D Returns orders created within the last week with any status.

💡 Hint

The first where filters by date, the second by status values.

The query filters orders created in the last week AND whose status is either 'shipped' or 'delivered'.

❓ state_output

expert

What is the count of records returned by this query?

Given the following data in Product table:
- 3 products with price 100
- 2 products with price 200
- 1 product with price 300

What is the result of:

Product.where('price >= ?', 150).where.not(price: 300).count

A 3

B 1

C 6

D 2

💡 Hint

Filter products with price >= 150, then exclude those with price 300.

Products with price >= 150 are those with prices 200 (2 products) and 300 (1 product). Excluding price 300 leaves 2 products.

Practice

1. What does the where method do in Rails ActiveRecord?

easy

A. Creates a new database table

B. Deletes records from the database

C. Updates all records without conditions

D. Filters records based on given conditions

Solution

1. Step 1: Understand the purpose of where

   The where method is used to specify conditions to filter records in a database query.

2. Step 2: Compare with other database actions

   Deleting, creating tables, or updating all records are different methods like delete, migrations, or update_all.

3. Final Answer:

   Filters records based on given conditions -> Option D

4. Quick Check:

   where filters records = C [OK]

Hint: Remember: where narrows down records by conditions [OK]

Common Mistakes:

• Confusing where with delete or update methods
• Thinking where creates or deletes tables
• Assuming where modifies all records without conditions

2. Which of the following is the correct syntax to find users with age 30 using where?

easy

A. User.where('age == ?', 30)

B. User.where(age: 30)

C. User.where(age == 30)

D. User.where(age = 30)

Solution

1. Step 1: Identify correct hash syntax for where

   Using a hash like age: 30 is the standard way to filter by attribute value.

2. Step 2: Check other options for syntax errors

   User.where('age == ?', 30) uses invalid SQL operator ==; User.where(age == 30) uses invalid Ruby comparison inside where; User.where(age = 30) uses assignment instead of comparison.

3. Final Answer:

   User.where(age: 30) -> Option B

4. Quick Check:

   Hash syntax for where = A [OK]

Hint: Use hash syntax for simple where conditions [OK]

Common Mistakes:

• Using == inside where instead of hash or string
• Using = instead of : or ? for conditions
• Confusing string and hash syntax

3. What will this code return?

User.where("name LIKE ?", "%John%")

medium

A. All users with name containing 'John'

B. All users with name exactly 'John'

C. Syntax error due to quotes

D. No users because LIKE is invalid

Solution

1. Step 1: Understand the SQL LIKE operator

   LIKE with % means match any characters before or after 'John', so it finds names containing 'John'.

2. Step 2: Check the where syntax

   The string with placeholder and argument is correct syntax for complex conditions.

3. Final Answer:

   All users with name containing 'John' -> Option A

4. Quick Check:

   LIKE with % means contains = A [OK]

Hint: LIKE with % means contains substring [OK]

Common Mistakes:

• Thinking LIKE matches exact string only
• Confusing syntax with invalid quotes
• Assuming LIKE is not supported in where

4. Identify the error in this code:

User.where(age > 25)

medium

A. Incorrect use of > inside where without string

B. Should use User.find instead of where

C. No error, code works fine

D. Missing quotes around condition string

Solution

1. Step 1: Check syntax of where conditions

   Conditions with operators like > must be inside a string or hash syntax; direct Ruby expressions are invalid.

2. Step 2: Identify correct usage

   Correct usage is User.where("age > ?", 25) or User.where("age > 25").

3. Final Answer:

   Incorrect use of > inside where without string -> Option A

4. Quick Check:

   Operators need string syntax in where = D [OK]

Hint: Use strings for operators like > in where [OK]

Common Mistakes:

• Writing Ruby expressions directly inside where
• Forgetting to quote conditions with operators
• Using find instead of where for filtering

5. How can you chain multiple where conditions to find users who are active and older than 18?

hard

A. User.where(active: true, age > ?, 18)

B. User.where('active = true AND age > ?')

C. User.where(active: true).where('age > ?', 18)

D. User.where(active == true).where(age > 18)

Solution

1. Step 1: Understand chaining of where

   Multiple where calls can be chained to combine conditions, each filtering further.

2. Step 2: Check syntax of each where call

   First where uses hash for simple boolean, second uses string with placeholder for age condition.

3. Step 3: Evaluate other options

   User.where(active: true, age > ?, 18) uses invalid Ruby expression without quotes; User.where('active = true AND age > ?') uses placeholder without bind argument; User.where(active == true).where(age > 18) uses invalid Ruby expressions.

4. Final Answer:

   User.where(active: true).where('age > ?', 18) -> Option C

5. Quick Check:

   Chain where with hash and string = B [OK]

Hint: Chain where calls for multiple conditions [OK]

Common Mistakes:

• Mixing hash and string in one where call incorrectly
• Using == inside where instead of : or placeholders
• Not chaining where calls properly