active record stuff originally drawn from https://speakerdeck.com/erniemiller/an-intervention-for-activerecord?slide=117

associations.rb

# use inverse_of to 'just work'
class User < ActiveRecord::Base
  has_many :contributions, inverse_of: :user
  has_many :posts, through: :contributions
end

class Post < ActiveRecord::Base
  has_many :contributions, inverse_of: :post
  has_many :contributors, through: :contributions,
    source: :user
end

class Contribution < ActiveRecord::Base
  belongs_to :user
  belongs_to :post
  validates :user, presence: true
  validates :post, presence: true
end

callbacks.rb

# prefer not to use callbacks
# if you do, have them only deal with the current class
# and not talk to other classes or relations
# or hit services or anything outside of itself

# and you can even get callback-like
# behavior as below which will always be called
# even when callbacks are not

# a pre-condition (before filter) 
# with a post-condition (after filter)
# that rollsback the transaction
def create_or_update
  say_my_name_before_filter &&
  super && say_goodbye_after_filter or raise ActiveRecord::Rollback
end

# like an around filter
# with a condition on the post part
def create_or_update
  puts 'we got' unless dont_do_it?
  super
  puts 'all the way around' if we_want_to?
end

initialization.rb

# always use after_initialize { }
# or after_initialize :something
# since def initialize(*) isn't always called

scoping.rb

# don't use default_scope
# it doesn't work

validations.rb

# don't validate e.g. association_id
# validate ruby objects!!

# UNIQUENESS
class Record < ActiveRecord::Base
  # validates :email, uniqueness: true
  # def save(*)
  #  super
  # rescue ActiveRecord::RecordNotUnique => e
  #  errors.add(:email, :taken)
  #  false
  # end
  validates :title,
    uniqueness: { scope: :subtitle }
  def save(*)
    super
  rescue ActiveRecord::RecordNotUnique => e
    attr_name, *scope = e.message.
      match(/Key \(([^\)]+)\)/)[1].split(/,\s*/)
    message = errors.generate_message(attr_name, :taken)
    if scope.any?
      message << " (scope: #{scope.to_sentence})"
    end
    errors.add attr_name, message
    false
  end
end