Active Record DB Majiayu000

Active Record & Databases: Rails ORM Mastery

Overview

Active Record is Rails' Object-Relational Mapping (ORM) layer. It connects Ruby objects to database tables, providing an elegant API for creating, reading, updating, and deleting data without writing SQL.

Active Record embodies Rails philosophy:

• Convention over configuration: Table names, foreign keys, and primary keys follow conventions
• DRY: Schema drives model attributes; no redundant declarations
• Object-oriented: Work with Ruby objects, not raw SQL
• Database agnostic: Same code works with PostgreSQL, MySQL, SQLite

Master Active Record and you master data in Rails applications.

Models and Conventions

Basic Model

A model represents a table and provides domain logic:

# app/models/product.rb
class Product < ApplicationRecord
  # Table: products
  # Primary key: id
  # Attributes: name, price, description, created_at, updated_at
end

Rails infers:

• Table name: products (pluralized)
• Primary key: id
• Attributes from schema
• Timestamps: created_at, updated_at

No configuration needed—just convention.

Naming Conventions

Irregular pluralizations work automatically:

• Person → people
• Child → children
• Octopus → octopi

Rails' inflector handles English pluralization rules.

Schema Conventions

Special column names have automatic behavior:

• id: Primary key (auto-generated)
• created_at: Set when record created
• updated_at: Updated when record saved
• lock_version: Optimistic locking counter
• type: Single Table Inheritance discriminator
• {association}_id: Foreign key for associations
• {association}_type: Polymorphic association type

Migrations

Migrations are Ruby scripts that modify database schema.

Creating Migrations

# Generate migration
rails generate migration CreateProducts name:string price:decimal

# Generate model (includes migration)
rails generate model Product name:string price:decimal

Generates:

# db/migrate/20240115100000_create_products.rb
class CreateProducts < ActiveRecord::Migration[8.0]
  def change
    create_table :products do |t|
      t.string :name
      t.decimal :price, precision: 10, scale: 2

      t.timestamps
    end
  end
end

Running Migrations

rails db:migrate              # Run pending migrations
rails db:rollback             # Undo last migration
rails db:migrate:status       # Show migration status
rails db:migrate VERSION=20240115100000  # Migrate to specific version

Migration Methods

Creating tables:

create_table :products do |t|
  t.string :name, null: false
  t.text :description
  t.decimal :price, precision: 10, scale: 2
  t.integer :quantity, default: 0
  t.boolean :available, default: true
  t.references :category, foreign_key: true
  t.timestamps
end

Modifying tables:

change_table :products do |t|
  t.rename :description, :details
  t.change :price, :decimal, precision: 12, scale: 2
  t.remove :quantity
  t.string :sku
  t.index :sku, unique: true
end

Adding columns:

add_column :products, :featured, :boolean, default: false
add_index :products, :name
add_reference :products, :supplier, foreign_key: true

Removing columns:

remove_column :products, :quantity
remove_index :products, :sku
remove_reference :products, :supplier

See references/migrations.md for comprehensive migration patterns.

Associations

Associations define relationships between models.

belongs_to

Declares a one-to-one or many-to-one relationship:

class Product < ApplicationRecord
  belongs_to :category
  # Expects: category_id column in products table
  # Provides: product.category
end

has_many

Declares a one-to-many relationship:

class Category < ApplicationRecord
  has_many :products
  # Expects: category_id column in products table
  # Provides: category.products
end

has_one

Declares a one-to-one relationship:

class User < ApplicationRecord
  has_one :profile
  # Expects: user_id column in profiles table
  # Provides: user.profile
end

has_many :through

Many-to-many with join model:

class Order < ApplicationRecord
  has_many :line_items
  has_many :products, through: :line_items
end

class LineItem < ApplicationRecord
  belongs_to :order
  belongs_to :product
end

class Product < ApplicationRecord
  has_many :line_items
  has_many :orders, through: :line_items
end

has_and_belongs_to_many

Many-to-many without join model:

class Product < ApplicationRecord
  has_and_belongs_to_many :tags
  # Expects: products_tags join table (no id, no timestamps)
end

class Tag < ApplicationRecord
  has_and_belongs_to_many :products
end

Polymorphic Associations

One model belongs to multiple model types:

class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
  # Expects: commentable_type and commentable_id columns
end

class Post < ApplicationRecord
  has_many :comments, as: :commentable
end

class Product < ApplicationRecord
  has_many :comments, as: :commentable
end

# Usage:
post.comments.create(body: "Great post!")
product.comments.create(body: "Love this product!")

See references/associations.md for advanced association patterns.

Querying

Active Record provides a rich query interface.

Finding Records

# Find by primary key
Product.find(1)
Product.find([1, 2, 3])

# Find by attributes
Product.find_by(name: "Widget")
Product.find_by!(name: "Widget")  # Raises if not found

# First, last, all
Product.first
Product.last
Product.all

Where Queries

# Simple conditions
Product.where(available: true)
Product.where("price < ?", 10)
Product.where("price BETWEEN ? AND ?", 10, 50)

# Hash conditions
Product.where(category_id: [1, 2, 3])
Product.where.not(category_id: 1)

# Ranges
Product.where(created_at: 1.week.ago..Time.now)

# Pattern matching
Product.where("name LIKE ?", "%widget%")

Ordering and Limiting

Product.order(created_at: :desc)
Product.order(price: :asc, name: :asc)
Product.limit(10)
Product.offset(20).limit(10)  # Pagination

Selecting Specific Columns

Product.select(:id, :name, :price)
Product.select("id, name, UPPER(name) as uppercase_name")

Joining Tables

# Inner join
Product.joins(:category)
Product.joins(:category, :tags)

# Left outer join
Product.left_outer_joins(:reviews)

# With conditions
Product.joins(:category).where(categories: { name: "Electronics" })

Eager Loading (N+1 Prevention)

Problem (N+1 queries):

products = Product.all
products.each do |product|
  puts product.category.name  # Fires query for EACH product!
end
# Fires: 1 query for products + N queries for categories = N+1 queries

Solution (eager loading):

products = Product.includes(:category).all
products.each do |product|
  puts product.category.name  # Uses preloaded data
end
# Fires: 1 query for products + 1 query for categories = 2 queries total

Methods:

• includes: Preload associations (two queries)
• eager_load: Preload with LEFT OUTER JOIN (one query)
• preload: Always uses separate queries

Scopes

Reusable query fragments:

class Product < ApplicationRecord
  scope :available, -> { where(available: true) }
  scope :cheap, -> { where("price < ?", 10) }
  scope :expensive, -> { where("price > ?", 100) }
  scope :in_category, ->(category) { where(category: category) }
end

# Usage:
Product.available.cheap
Product.expensive.in_category("Electronics")

Method Chaining

Build complex queries incrementally:

products = Product.all

products = products.where(available: true) if params[:available]
products = products.where(category: params[:category]) if params[:category]
products = products.where("price < ?", params[:max_price]) if params[:max_price]

products = products.order(params[:sort] || :created_at)
products = products.page(params[:page])

products  # Execute query when enumerated

Validations

Ensure data integrity before saving.

Common Validations

class Product < ApplicationRecord
  validates :name, presence: true
  validates :price, numericality: { greater_than: 0 }
  validates :sku, uniqueness: true
  validates :email, format: { with: URI::MailTo::EMAIL_REGEXP }
  validates :description, length: { minimum: 10, maximum: 500 }
  validates :category, presence: true
  validates :terms, acceptance: true
end

Conditional Validations

validates :coupon_code, presence: true, if: :coupon_used?
validates :shipping_address, presence: true, unless: :pickup?

Custom Validations

validate :price_must_be_reasonable

private

def price_must_be_reasonable
  if price.present? && price > 10000
    errors.add(:price, "is unreasonably high")
  end
end

Validation Helpers

# Inline validation
product.valid?  # => false
product.errors.full_messages  # => ["Name can't be blank", "Price must be greater than 0"]

# Save with validation
product.save  # => false (doesn't save if invalid)
product.save!  # => raises ActiveRecord::RecordInvalid

# Skip validation (dangerous!)
product.save(validate: false)

Callbacks

Run code at specific points in an object's lifecycle.

Common Callbacks

class Product < ApplicationRecord
  before_validation :normalize_name
  after_validation :log_errors
  before_save :calculate_discount
  after_save :clear_cache
  before_create :generate_sku
  after_create :notify_team
  before_update :track_price_changes
  after_update :reindex_search
  before_destroy :check_orders
  after_destroy :cleanup_images
  after_commit :sync_to_external_system

  private

  def normalize_name
    self.name = name.strip.titleize if name.present?
  end

  def generate_sku
    self.sku = SecureRandom.hex(8).upcase
  end

  def check_orders
    throw :abort if orders.exists?
  end
end

Callback Order

1. before_validation
2. after_validation
3. before_save
4. before_create / before_update
5. Database operation
6. after_create / after_update
7. after_save
8. after_commit / after_rollback

Skipping Callbacks

product.update_columns(price: 9.99)  # Skips callbacks and validations
product.update_attribute(:price, 9.99)  # Skips validations only
product.increment!(:view_count)  # Skips validations, runs callbacks

Advanced Patterns

Single Table Inheritance (STI)

class Vehicle < ApplicationRecord
  # type column required
end

class Car < Vehicle
end

class Truck < Vehicle
end

# Queries:
Car.all  # WHERE type = 'Car'
Vehicle.all  # All vehicles

Enums

class Order < ApplicationRecord
  enum status: [:pending, :processing, :shipped, :delivered, :cancelled]
end

order = Order.create!(status: :pending)
order.pending?  # => true
order.processing!  # Updates status to processing
order.processing?  # => true

Order.pending  # WHERE status = 0
Order.not_pending  # WHERE status != 0

Composite Primary Keys (Rails 8)

class BookOrder < ApplicationRecord
  self.primary_key = [:book_id, :order_id]

  belongs_to :book
  belongs_to :order
end

# Find by composite key
BookOrder.find([book_id, order_id])

Database-Specific Features

PostgreSQL

# JSON columns
add_column :products, :metadata, :jsonb, default: {}

product.metadata = { color: "red", size: "large" }
Product.where("metadata->>'color' = ?", "red")

# Arrays
add_column :products, :tags, :string, array: true, default: []

product.tags = ["electronics", "sale"]
Product.where("? = ANY(tags)", "electronics")

# Full-text search
Product.where("to_tsvector('english', name) @@ to_tsquery(?)", "widget")

MySQL-Specific

# Case-insensitive queries (default)
Product.where("name = ?", "Widget")  # Matches "widget", "WIDGET"

# JSON columns (MySQL 5.7+)
add_column :products, :settings, :json

Best Practices

1. Use scopes for reusable queries
2. Eager load to prevent N+1 queries
3. Add indexes for foreign keys and frequently queried columns
4. Validate before saving to maintain data integrity
5. Use transactions for multi-step operations
6. Limit callbacks - keep them simple and focused
7. Use migrations - never modify schema directly
8. Test validations and associations
9. Profile queries - use explain to optimize
10. Use database constraints (NOT NULL, UNIQUE, FOREIGN KEY)

Further Reading

For deeper exploration:

• references/migrations.md: Complete migration guide with patterns
• references/associations.md: Advanced association techniques
• references/query-optimization.md: Performance tuning and N+1 prevention

For code examples:

• examples/active-record-patterns.rb: Common Active Record patterns

Summary

Active Record provides:

• Models that represent database tables
• Migrations for schema changes
• Associations for relationships
• Validations for data integrity
• Queries without writing SQL
• Callbacks for lifecycle hooks
• Conventions that eliminate configuration

Master Active Record, and you master data in Rails.