useful_active_record.rb

User.find(10) 
=> #<User id: 10, name: => "Tom">

User.find(1, 10) #or ([1, 10])
=> #[<User id: 1, name: => "Lifo">, #<User id: 10, name: => "Ryan">]
  
User.first
User.last
  
User.all

#User.all makes Active Record fetch the entire table, build a model object per row, 
#and keep the entire array in the memory. find_each can iterate over data in a
#table in batches which is useful with tables of thousands of rows
User.find_each(:batch_size => 5000) do |user|
  NewsLetter.weekly_deliver(user)
end

#Checks if User with an id exists
User.exists?(1,2,3)

#All SQL calculations can be used with Active Record methods
User.count
User.average("age")
User.minimum("age")
User.maximum("age")
User.sum("age")

# .where finds based on a condition
User.where("age = 20")

#Group by a specific characteristic
User.group(:name)

#Order by a characteristic
User.order(:name)

#Run pure SQL using Active Record
User.find_by_sql("SELECT * FROM users WHERE age > 20")