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EAGER LOADING

Eager loading in Rails is a technique used to optimize database queries by loading associated records of the objects returned by the query, thereby reducing the number of database calls. This is particularly useful when you have associations defined in your models and you want to avoid the "N+1 query problem."

What is the N+1 Query Problem?

“N+1 Queries” are a very common cause of repeated queries in Rails applications. This happens when you make a request for a single row in one table, and then make an additional request per element in a has_many relationship, usually in a loop.
Here’s an example:

Consider two models: Author and Book, where an Author has many Books.

class Author < ApplicationRecord 
  has_many :books 
 end 

class Book < ApplicationRecord 
  belongs_to :author 
end 

Scenario Without Eager Loading
When you fetch authors and then access their books, Rails performs an additional query for each author to get their books.

# Fetching all authors and their books 
authors = Author.all 

authors.each do |author| 
  puts author.books.pluck(:title) 
end 

This will result in one query to fetch the authors and multiple queries to fetch books for each author, leading to the N+1 query problem.

How Eager Loading Works

Eager loading addresses this problem by loading all the necessary data in as few queries as possible, usually through the use of includes, eager_load, or preload.

Using Eager Loading with includes

To avoid the N+1 query problem, you can use includes to preload the associated books when fetching authors.

# Fetching all authors and their books with eager loading 
authors = Author.includes(:books) 

authors.each do |author| 
  puts author.books.pluck(:title) 
end 

With includes, Rails performs a single query to fetch all authors and another single query to fetch all associated books.

Using Eager Loading with eager_load

eager_load forces Rails to use a SQL JOIN to load the associated records. This can be useful if you need to filter or sort based on the associated records.

# Fetching all authors and their books with eager loading using JOIN 
authors = Author.eager_load(:books) 

authors.each do |author| 
  puts author.books.pluck(:title) 
end 

This approach performs a single query with a LEFT OUTER JOIN.

Using Eager Loading with preload

preload is similar to includes, but it always uses separate queries for loading the associations.

# Fetching all authors and their books with eager loading using separate queries 
authors = Author.preload(:books) 

authors.each do |author| 
  puts author.books.pluck(:title) 
end 

preload is useful when you know that separate queries will be more efficient, for example, when fetching a large number of records.

Summary

Eager loading is a powerful tool in Rails that helps to optimize database access by pre-loading associations. Using includes, eager_load, or preload appropriately can significantly improve the performance of your application by reducing the number of database queries.

#rails #db #optimizing_queries

Scoped Associations with Joins

Let's consider the following models:
• Author with attributes id, name
• Book with attributes id, title, published, author_id
The association is that an Author has many Books, and a Book belongs to an Author.

Defining Scoped Associations
First, let's define a scope on the Book model to filter only published books:

class Book < ApplicationRecord 
  belongs_to :author 
  scope :published, -> { where(published: true) } 
end 

Next, let's use this scope in the Author model to create a scoped association:

class Author < ApplicationRecord 
  has_many :books 
  has_many :published_books, -> { published }, class_name: 'Book' 
end 

Now, Author has an association called published_books, which only includes books that are published.

Using Scoped Associations with Joins

We can now use the scoped association in joins to fetch authors and their published books.
Example 1: Fetch Authors with Published Books
To fetch authors along with their published books:

@authors_with_published_books = Author.joins(:published_books).select('authors.*, books.title as book_title') 

This query joins the authors table with the books table but only includes books that are published, thanks to the scoped association published_books.

As a result, each Author object in @authors_with_published_books will have:
• All attributes of the Author model.
• An additional attribute called book_title representing the title of each associated book.
#rails #scoped_associations_with_joins

Scoped associations

Let's say we have two models: User and Post. And we want to get all posts for user. We can do this:

class User < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :user
end

user = User.first
user.posts

But what if we don't ever want to include posts for user which are published? Often I see people doing this:

class User < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :user

  scope :published, -> { where(published: true) }
end

user = User.first
user.posts.published

And this is fine, but you can somehow forget to add published scope to your query. And then you will get all posts, even unpublished. And this is not what you want. Some people use default_scope for this, but it is not good idea. So what we can do? We can use scope in association:

class User < ActiveRecord::Base
  has_many :posts, -> { published }
  has_many :not_published_posts, -> { not_published }
end

class Post < ActiveRecord::Base
  belongs_to :user

  scope :published, -> { where(published: true) }
  scope :not_published, -> { where(published: false) }
end

user = User.first
user.posts # only published posts here

#rails #database #model #associations

There are two ways to call respond_to: Either we can pass it as a list of symbols or pass a block
Block Version:

def index
  @people = Person.all

  respond_to do |format|
    format.json { render json: @people}
    format.xml { render xml: @people }
    format.html { render :index}
  end
end

Symbol version:

def index
  @people = Person.all
  respond_to :json, :xml, :html  
end

#respondto #actionview #rails

kubectl rollout restart deployment <deployment-name> command used to restart pods managed by a Kubernetes deployment without making any modifications to the deployment configuration.
#devops #Kubernetes #rollingrestart

Active Record is the Object-Relational Mapping (ORM) layer provided by Ruby on Rails. It facilitates the interaction between Ruby objects and a relational database. It helps us interact with our database using Ruby instead of writing SQL queries directly.

Key features of ActiveRecord include:

• Model Definition: ActiveRecord allows developers to define models that represent database tables. These models inherit from the ActiveRecord::Base class and include methods for defining associations, validations, and callbacks.
• CRUD Operations: ActiveRecord provides methods for performing CRUD (Create, Read, Update, Delete) operations on database records. These methods allow developers to manipulate records using Ruby syntax without writing explicit SQL queries.
• Associations: ActiveRecord simplifies the definition and management of associations between models. It supports various types of associations, including belongs_to, has_many, has_one, and has_and_belongs_to_many, allowing us to express complex relationships between database tables.
• Validations: ActiveRecord includes a robust validation framework for ensuring data integrity. We can define validation rules such as presence, uniqueness, length, format, and custom validations to enforce data constraints at the model level.
#orm #rails

What is happening behind the scenes after we configured database credentials in database.yml
• Conduct the three-way handshake to establish a TCP connection to the server.
• Exchange preferences and requirements with the database software to establish the session parameters.
• Perform database authentication checks to establish the client's identity.
• PostgreSQL supports several authentication methods, such as passwords (plaintext or MD5),GSSAPI, SSPI, and more.
• The client (Rails, via ActiveRecord) can now send commands to the server. Commands are typically SQL statements. These statements are sent as simple text strings in the 'Query' message format.
How Number of connection affects the database server
• When the number of connections or pools to a server increases, CPU usage will rise. This increased demand for memory and CPU resources can affect other operations, such as transaction speeds. Managing many connections can also reduce the effectiveness of the database and decreasing overall system performance.
What will happen if we give more connections then we configured in PSQL?
• When that limit of a db is reached, additional connection requests are rejected.
How connections b/w rails and PSQL established and maintained?
• Connection pooling is a technique where database connections are reused for multiple requests instead of being closed after each query. This approach uses a connection pooler, a software that manages the connections between the database and its client applications, optimizing the use of resources and improving performance.
• Establishing the connections is a fairly long operation.
• Connection pooler is sitting b/w client and server. client connects to the connection pooler instead of directly to the database. Req is sent to pooler and the pooler interprets queries returns back the response from DB.
• Once the response is returned it closes the connection. But the thing here is opening and closing connection is not overhead comparatively with client and DB connections.
#orm #activerecord #psql #rails

Dup vs clone while dealing with Active Record:
• Dup creates a new record with a blank ID, which will create a new object on the database with a new ID after hitting .save.

user = User.first
user1 = user.dup
user1:
<User id: nil, name: "Rails"> 

• clone creates a new record with the same ID, after hitting save on the clone object this will overwrite the existing data.

user = User.first
user1 = user.clone
<User id: 1, name: "Rails"> 

• If you change some value on the above attribute and put .save it will overwrite the original obj.
#dupvsclone #ruby #rails

In Rails, the resources method provides a convenient way to define RESTful routes for our application's resources. Instead of manually specifying separate routes for each CRUD action (Create, Read, Update, Delete), we can use the resources method to define all these routes with a single line of code.

# config/routes.rb

Rails.application.routes.draw do
  resources :students
end

With this single line of code, Rails automatically generates the following RESTful routes for the students resource:

GET     /students          # Index action (display a list of students)
GET     /students/new      # New action (display a form to create a new student)
POST    /students          # Create action (create a new student)
GET     /students/:id      # Show action (display details of a specific student)
GET     /students/:id/edit # Edit action (display a form to edit a specific student)
PATCH   /students/:id      # Update action (update a specific student)
PUT     /students/:id      # Update action (update a specific student)
DELETE  /students/:id      # Destroy action (delete a specific student)

#rails #routes

In a Rails application, the seeds.rb file is used to populate the database with initial data. This file typically resides in the db directory of our Rails application. The data added through the seeds.rb file is often used for testing or for providing initial data in a fresh installation of the application.

After defining the data in the seeds.rb file, we can populate the database by running the rails db:seed
#rails #database