Active Record pattern in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Active Record pattern in Rails?

It is a design pattern where each model class wraps a database table and provides methods to create, read, update, and delete records. It connects objects and database rows directly.

beginner

How does Active Record in Rails simplify database interactions?

It lets you work with Ruby objects instead of writing SQL queries. You can call methods like .find, .save, and .destroy on model instances to manage data.

beginner

What method would you use to get all records of a model in Active Record?

You use the .all method. For example, User.all returns all users from the users table as Ruby objects.

intermediate

Explain the relationship between a Rails model and the database in the Active Record pattern.

Each Rails model corresponds to a database table. Each instance of the model represents a row in that table. The model handles data validation, querying, and persistence.

intermediate

What happens when you call save on an Active Record model instance?

If the object is new, it inserts a new row in the database. If it already exists, it updates the existing row with any changed attributes.

In Rails Active Record, which method fetches a record by its ID?

A .get

B .fetch

C .find

D .select

The .find method retrieves a record by its primary key (usually ID).

What does User.new do in Active Record?

A Creates a new user object but does not save it to the database

B Fetches a user from the database

C Deletes a user record

D Updates a user record

User.new creates a new instance in memory but does not save it until save is called.

Which Active Record method removes a record from the database?

A .destroy

B .remove

C .delete

D .erase

.destroy deletes the record and runs callbacks, while .delete removes it without callbacks.

How does Active Record know which database table to use for a model named Article?

A You must specify the table manually

B It uses the pluralized lowercase name: articles

C It uses the model name as is: Article

D It uses a random table

Active Record automatically maps Article to the articles table by pluralizing and lowercasing the model name.

What is the main benefit of using the Active Record pattern in Rails?

A It disables validations

B It requires writing raw SQL for every query

C It only works with NoSQL databases

D It hides database details and lets you work with Ruby objects

Active Record abstracts database access so you can use Ruby code instead of SQL, making development easier and safer.

Describe how the Active Record pattern connects Ruby objects to database tables in Rails.

Think about how a model and its instances relate to the database.

Explain what happens when you create a new model instance and call save in Rails Active Record.

Focus on the lifecycle of a model object from creation to database storage.

Practice

1. What is the main purpose of the Active Record pattern in Rails?

easy

A. To link Ruby classes with database tables for easy data management

B. To create user interfaces with Ruby code

C. To handle HTTP requests and responses

D. To style web pages using CSS

Solution

1. Step 1: Understand Active Record's role

   Active Record connects Ruby classes to database tables, making data handling easier.

2. Step 2: Compare options to this role

   Only To link Ruby classes with database tables for easy data management describes this connection; others describe unrelated tasks.

3. Final Answer:

   To link Ruby classes with database tables for easy data management -> Option A

4. Quick Check:

   Active Record = Ruby class to DB table link [OK]

Hint: Active Record links Ruby classes to tables [OK]

Common Mistakes:

• Confusing Active Record with view or controller roles
• Thinking it handles HTTP or styling
• Assuming it writes SQL directly

2. Which of the following is the correct way to find a record by its ID using Active Record?

easy

A. User.find_by_id(5)

B. User.where(id: 5).first!

C. User.find(5)

D. User.get(5)

Solution

1. Step 1: Recall Active Record find methods

   The method find fetches a record by its ID directly.

2. Step 2: Check each option

   User.find(5) uses find(5), which is correct. User.find_by_id(5) is deprecated. User.where(id: 5).first! is valid but less direct. User.get(5) is invalid.

3. Final Answer:

   User.find(5) -> Option C

4. Quick Check:

   Use find(id) to get record by ID [OK]

Hint: Use find(id) to get record by ID [OK]

Common Mistakes:

• Using deprecated find_by_id instead of find
• Using invalid method names like get
• Confusing where.first! with find for simple ID lookup

3. Given the code:

user = User.find(3)
user.update(name: "Alice")
puts user.name

What will be printed?

medium

A. Alice

B. An error message

C. nil

D. 3

Solution

1. Step 1: Understand the update method

   The update method changes the record's attributes and saves it.

2. Step 2: Check the output of puts user.name

   After updating, user.name is "Alice", so it prints "Alice".

3. Final Answer:

   Alice -> Option A

4. Quick Check:

   update changes attribute, then prints new value [OK]

Hint: update changes and saves record attributes [OK]

Common Mistakes:

• Thinking update returns the ID
• Assuming update does not save changes
• Expecting nil or error without reason

4. What is wrong with this code snippet?

post = Post.find(10)
post.title = "New Title"
post.save!

medium

A. Using save! will not save changes

B. No error; this code correctly updates the post title

C. Assigning title directly is not allowed

D. find(10) returns an array, not a single record

Solution

1. Step 1: Analyze the code actions

   The code finds a post by ID, changes its title, then saves it with save!.

2. Step 2: Check if any step is invalid

   Assigning attributes directly and using save! is valid and will update the record.

3. Final Answer:

   No error; this code correctly updates the post title -> Option B

4. Quick Check:

   Direct attribute assignment + save! updates record [OK]

Hint: Direct attribute change + save! updates record [OK]

Common Mistakes:

• Thinking save! does not save changes
• Believing find returns an array
• Assuming direct assignment is forbidden

5. You want to get all users who signed up after January 1, 2023, and order them by their signup date descending. Which Active Record query is correct?

hard

A. User.select_all.where(created_at > '2023-01-01').order_desc('created_at')

B. User.find_by(created_at: '> 2023-01-01').sort(created_at: :desc)

C. User.filter(created_at > '2023-01-01').order_by('created_at desc')

D. User.where('created_at > ?', '2023-01-01').order(created_at: :desc)

Solution

1. Step 1: Identify correct where syntax

   Active Record uses where('created_at > ?', date) to filter by date.

2. Step 2: Identify correct order syntax

   Ordering is done with order(created_at: :desc) for descending order.

3. Step 3: Compare options

   Only User.where('created_at > ?', '2023-01-01').order(created_at: :desc) uses correct where and order syntax; others use invalid or non-existent methods.

4. Final Answer:

   User.where('created_at > ?', '2023-01-01').order(created_at: :desc) -> Option D

5. Quick Check:

   where + order with symbols = correct query [OK]

Hint: Use where with ? and order with symbol for sorting [OK]

Common Mistakes:

• Using find_by instead of where for multiple records
• Using non-existent methods like filter or order_desc
• Incorrect syntax in where clause