# Create
user = User.create(name: "Alice", email: "alice@example.com")
user = User.create!(...)  # Raises on failure

# Read
User.find(1)              # Raises RecordNotFound
User.find_by(email: "x")  # Returns nil if not found
User.where(active: true)  # Returns Relation

# Update
user.update(name: "Bob")
user.update!(...)         # Raises on failure

# Delete (callbacks run)
user.destroy

# Delete (no callbacks)
user.delete

Need to access associated data?
├── NO → Use `joins` (filtering only)
└── YES → Need to filter/sort by association?
          ├── NO → Use `preload` (separate queries)
          └── YES → Large dataset with many associations?
                    ├── YES → Use `includes` with `references`
                    └── NO → Use `eager_load` (single JOIN)

# N+1 problem
Post.all.each { |p| p.author.name }  # 1 + N queries

# Solution
Post.includes(:author).each { |p| p.author.name }  # 2 queries

Does the rule ALWAYS apply, regardless of business logic?
├── Yes → Database constraint
│   └── Examples: NOT NULL, foreign keys, unique emails
└── No → Model validation
    └── Examples: Format rules that change, conditional requirements

Need helpful user-facing error messages?
├── Yes → Model validation (possibly WITH constraint)
└── No → Constraint alone is fine

# Migration (data integrity)
add_index :users, :email, unique: true

# Model (user feedback)
validates :email, presence: true, uniqueness: true

class Author < ApplicationRecord
  has_many :books, dependent: :destroy
  has_one :profile
end

class Book < ApplicationRecord
  belongs_to :author                    # Required by default
  belongs_to :publisher, optional: true # Allow NULL
end

class Physician < ApplicationRecord
  has_many :appointments
  has_many :patients, through: :appointments
end

class Appointment < ApplicationRecord
  belongs_to :physician
  belongs_to :patient
  # Join model can have attributes
  validates :scheduled_at, presence: true
end

# Always reversible
add_column :users, :name, :string
add_index :users, :email, unique: true
add_reference :orders, :user, foreign_key: true

# Concurrent index (no table lock)
disable_ddl_transaction!
add_index :users, :email, algorithm: :concurrently

remove_column :users, :legacy_field, :string  # Include type!
change_column_default :users, :status, from: nil, to: "active"

before_validation → after_validation →
before_save → around_save → before_create →
around_create → [INSERT] → after_create →
after_save → [COMMIT] → after_commit

# WRONG - Race condition!
after_save :enqueue_processing

# CORRECT - Runs after COMMIT
after_commit :enqueue_processing, on: :create

# BAD - loads all records
User.all.each { |u| process(u) }

# GOOD - processes in batches
User.find_each { |u| process(u) }

# Bulk operations
User.where(old: true).in_batches.update_all(archived: true)