21. Active Record Basics

1# Table: users
2# | id | name  | email           | created_at |
3# |----|-------|-----------------|------------|
4# | 1  | John  | john@email.com  | 2024-01-15 |
5# | 2  | Jane  | jane@email.com  | 2024-01-16 |
6
7# Ruby class
8class User < ApplicationRecord
9end
10
11# Object
12user = User.find(1)
13user.name   # => "John"
14user.email  # => "john@email.com"

1rails generate model User name:string email:string age:integer active:boolean
2
3# Creates:
4# app/models/user.rb
5# db/migrate/xxx_create_users.rb
6# test/models/user_test.rb

1# app/models/user.rb
2class User < ApplicationRecord
3  # Model code goes here
4end

1# db/migrate/20240115000000_create_users.rb
2class CreateUsers < ActiveRecord::Migration[7.1]
3  def change
4    create_table :users do |t|
5      t.string :name
6      t.string :email
7      t.integer :age
8      t.boolean :active, default: true
9
10      t.timestamps  # Adds created_at and updated_at
11    end
12  end
13end

1class User < ApplicationRecord
2  self.table_name = "my_users"
3  self.primary_key = "user_id"
4end

1# Method 1: new + save
2user = User.new
3user.name = "John"
4user.email = "john@example.com"
5user.save  # Returns true/false
6
7# Method 2: new with hash
8user = User.new(name: "John", email: "john@example.com")
9user.save
10
11# Method 3: create (new + save in one step)
12user = User.create(name: "John", email: "john@example.com")
13
14# Method 4: create! (raises exception on failure)
15user = User.create!(name: "John", email: "john@example.com")

1# Find by ID
2user = User.find(1)           # Raises error if not found
3user = User.find_by(id: 1)    # Returns nil if not found
4
5# Find by attribute
6user = User.find_by(email: "john@example.com")
7user = User.find_by!(email: "john@example.com")  # Raises if not found
8
9# Find all
10users = User.all
11
12# Find with conditions
13users = User.where(active: true)
14users = User.where("age > ?", 18)
15users = User.where("name LIKE ?", "%John%")
16
17# First and last
18first_user = User.first
19last_user = User.last
20
21# Count
22User.count
23User.where(active: true).count

1# Method 1: Find, modify, save
2user = User.find(1)
3user.name = "Jane"
4user.save
5
6# Method 2: update
7user = User.find(1)
8user.update(name: "Jane")
9
10# Method 3: update! (raises on failure)
11user.update!(name: "Jane")
12
13# Update all matching records
14User.where(active: false).update_all(active: true)
15
16# Update single attribute (skips validations)
17user.update_attribute(:name, "Jane")
18
19# Update column directly (skips callbacks and validations)
20user.update_column(:name, "Jane")

1# Delete a single record (runs callbacks)
2user = User.find(1)
3user.destroy
4
5# Delete by ID
6User.destroy(1)
7
8# Delete all matching
9User.where(active: false).destroy_all
10
11# Delete without callbacks
12user.delete
13User.delete(1)
14User.where(active: false).delete_all

1user = User.new
2
3user.new_record?   # => true (not saved yet)
4user.persisted?    # => false
5
6user.save
7
8user.new_record?   # => false
9user.persisted?    # => true
10
11user.changed?      # => false
12user.name = "New Name"
13user.changed?      # => true
14user.name_changed? # => true
15user.changes       # => {"name"=>["John", "New Name"]}
16
17# Check if record exists
18User.exists?(1)
19User.exists?(email: "john@example.com")
20User.where(active: true).exists?

1user = User.find(1)
2# Someone else updates the record in the database
3user.reload  # Refresh from database

1user = User.find(1)
2
3# Get all attributes as a hash
4user.attributes
5# => {"id"=>1, "name"=>"John", "email"=>"john@example.com", ...}
6
7# Access attributes
8user[:name]
9user.name
10user.read_attribute(:name)
11
12# Set attributes
13user[:name] = "Jane"
14user.name = "Jane"
15user.write_attribute(:name, "Jane")
16
17# Assign multiple (doesn't save)
18user.assign_attributes(name: "Jane", email: "jane@example.com")
19user.save

1# timestamps are automatically added
2class CreatePosts < ActiveRecord::Migration[7.1]
3  def change
4    create_table :posts do |t|
5      t.string :title
6      t.datetime :published_at
7      t.date :event_date
8      t.time :start_time
9      
10      t.timestamps  # created_at and updated_at
11    end
12  end
13end
14
15# Using date/time
16post = Post.create(
17  title: "My Post",
18  published_at: Time.current,
19  event_date: Date.today,
20  start_time: Time.current
21)
22
23post.created_at  # When created
24post.updated_at  # When last updated (auto-updated)

1user.touch                    # Update updated_at to now
2user.touch(:last_login_at)    # Update specific timestamp column

1# Increment counter
2article.increment(:views_count)
3article.increment!(:views_count)  # Saves immediately
4
5# Decrement
6product.decrement(:stock)
7product.decrement!(:stock)