comment.rb

# As a first example, let's consider an "adjacency list", a tree represented in a table. Suppose we have a table comments, representing a threaded discussion:

comments = Arel::Table.new(:comments)
# [:id, :body, :parent_id]

replies = comments.alias
comments_with_replies = \
  comments.join(replies).on(replies[:parent_id].eq(comments[:id]))
# => SELECT * FROM comments INNER JOIN comments AS comments_2 WHERE comments_2.parent_id = comments.id

product.rb

# Suppose we have a table products with prices in different currencies. And we have a table currency_rates, of constantly changing currency rates.

# In Arel:

products = Arel::Table.new(:products)
# Attributes: [:id, :name, :price, :currency_id]

currency_rates = Arel::Table.new(:currency_rates)
# Attributes: [:from_id, :to_id, :date, :rate]

products.
  join(:currency_rates).on(products[:currency_id].eq(currency_rates[:from_id])).
  where(currency_rates[:to_id].eq(user_preferred_currency), currency_rates[:date].eq(Date.today)).
  order(products[:price] * currency_rates[:rate])

profile_group_memberships.rb

class ProfileGroupMemberships < Struct.new(:user, :visitor)
  def groups
    @groups ||= user.groups.where public.or sharedmembership
  end
  
private

  def public
    table[:private].eq false
  end
  
  def shared_membership
    table[:id].in visitor.group_id
  end

  def table
    Group.arel_table
  end
end

user.rb

users.where users[:name].eq 'amy'
# User.where(name: 'amy')

users.project users[:id]
# User.select :id

users.join(photos).on users[:id].eq photos[:user_id]
# 

users.take(5)
# User.limit(5)

users.skip(4)
# User.offset(4)

users
  .where(users[:name].eq('amy'))
  .project(users[:id])
  
User.where(name: 'amy').select(:id)

users.where(users[:name].eq('bob')).where(users[:age].lt(25))
User.where(name: 'bob').where('age < ?', 25)

users.where(users[:name].eq('bob').or(users[:age].lt(25)))
User.where('name IS :name OR age < :age', name: 'bob', age: 25)