convert select and collect methods into joins

问题

I am learning joins I have three models

class Booking < ActiveRecord::Base
  has_and_belongs_to_many :groups
  has_and_belongs_to_many :users
end

class User < ActiveRecord::Base
  belongs_to :group
  has_and_belongs_to_many :bookings
end

class Group < ActiveRecord::Base
  has_many :users
  has_and_belongs_to_many :bookings
end

booking = Booking.first
booking.groups.collect{|g| g.users.select{|u| !self.users.include? u}}.flatten

I want to get all users which are not booked in booking groups some thing like this

booking=Booking.first
booking.groups.collect{|g| g.users.select{|u| !self.users.include? u}}.flatten

How can I do it with joins instead collect and select?

回答1:

It should be something like (using arel gem):

class User < ActiveRecord::Base
   scope :non_booked_for, ->(booking_id) {
      users = User.arel_table
      bookings = Booking.arel_table
      groups = Group.arel_table
      where(groups_bookings[:booking_id].eq(booking_id)
       .and(groups_bookings[:group_id].eq(users[:group_id])))
       .not.where(user_bookings[:user_id].eq(users[:id])
             .and(user_bookings[:booking_id].eq(booking_id)))
   end
end

usage:

users = User.non_booked_for Booking.first.id

It just uses negation to all users, which belong to group and bookings, and its group belongs to that booking too. But I'm not sure that the negation will work properly here, so it should be debugged. And please select on of the two table names: users_bookings, or bookings_users for the associations, so, for example, in models it will lead to:

has_and_belongs_to_many :users

has_and_belongs_to_many :bookings, table_name: :users_bookings

来源：https://stackoverflow.com/questions/27542188/convert-select-and-collect-methods-into-joins

标签

mysql

ruby-on-rails

activerecord