Where conditions in Ruby on Rails - Mini Project: Build & Apply

Using Where Conditions in Rails

📖 Scenario: You are building a simple Rails app to manage a bookstore's inventory. You want to find books based on certain conditions like price and availability.

🎯 Goal: Learn how to use where conditions in Rails ActiveRecord queries to filter records from the database.

📋 What You'll Learn

Create a Book model with specific attributes

Add a variable to hold a price limit

Use where with a condition to find books cheaper than the price limit

Add a final query to find books that are both cheap and in stock

💡 Why This Matters

🌍 Real World

Filtering database records by conditions is common in web apps to show users only relevant data.

💼 Career

Knowing how to use ActiveRecord <code>where</code> queries is essential for Rails developers to build efficient and user-friendly applications.

1

Create the Book model with attributes

Create a Book model with these exact attributes: title:string, price:integer, and in_stock:boolean.

Ruby on Rails

# Your code here to define Book model with title, price, and in_stock attributes

Hint

In Rails, models inherit from ApplicationRecord. Attributes are defined in the database migration, but for this exercise, just define the model class.

2

Add a price limit variable

Add a variable called max_price and set it to 20 to represent the maximum price for filtering books.

Ruby on Rails

class Book < ApplicationRecord
end

# Your code here to add max_price variable

Hint

Just create a simple variable named max_price and assign the number 20.

3

Use where to find books cheaper than max_price

Write a query called cheap_books that uses Book.where to find books with price less than max_price.

Ruby on Rails

class Book < ApplicationRecord
end

max_price = 20

# Your code here to find cheap_books using where condition

Hint

Use where("price < ?", max_price) to filter books cheaper than max_price.

4

Find books that are cheap and in stock

Write a query called available_cheap_books that uses Book.where with conditions to find books where price is less than max_price and in_stock is true.

Ruby on Rails

class Book < ApplicationRecord
end

max_price = 20

cheap_books = Book.where("price < ?", max_price)

# Your code here to find available_cheap_books with two conditions

Hint

Use where("price < ? AND in_stock = ?", max_price, true) to filter books that are cheap and in stock.

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