Aliasing the source table

Question

Is there a way to alias the source table in the context of a single scope ?

I tried this :

scope = User.all
scope.arel.source.left.table_alias = "toto"
scope.where(firstname: nil) => "SELECT `toto`.* FROM `users` `toto` WHERE `toto`.`firstname` IS NULL"

The problem is that the model class keeps the alias for all subsequent queries :

User.all => "SELECT `toto`.* FROM `users` `toto`"

Edit

I added this method to ApplicationRecord

def self.alias_source(table_alias)
  klass = Class.new(self)
  klass.all.source.left.table_alias = table_alias
  klass
end

Now, I can do :

User.alias_source(:toto).where(firstname: nil) => "SELECT `toto`.* FROM `users` `toto` WHERE `toto`.`firstname` IS NULL"

Answer 1

Since creating Anonymous Classes that inherit from ActiveRecord::Base will cause memory bloat, I am not sure I would recommend it (See Here: https://github.com/rails/rails/issues/31395).

Maybe a better implementation would be to execute in a block form instead e.g. yield the aliased table to the block then set it back afterwards?

e.g.

def self.with_table_alias(table_alias, &block)
  begin 
    self.all.source.left.table_alias = table_alias
    block.call(self)
  ensure 
    self.all.source.left.table_alias = nil 
  end
end

Usage

User.with_table_alias(:toto) do |scope| 
  puts scope.joins(:posts).where(firstname: "Ruur").to_sql
end 
# "SELECT `toto`.* 
#  FROM 
#    `users` `toto` 
#    INNER JOIN `posts` ON `posts`.`user_id` = `toto`.`id` 
#  WHERE 
#    `toto`.`firstname` = 'Ruur'" 

puts User.all.to_sql
# "SELECT `users`.* FROM `users`

WARNING: THIS HAS NOT BE TESTED BEYOND WHAT IS SHOWN HERE AND I WOULD NOT RECOMMEND THIS IN A PRODUCTION ENVIRONMENT WITHOUT EXTENSIVE TESTING FOR EDGE CASES AND OTHER GOTCHAS

Update To address desired implementation

module ARTableAlias

  def alias_table_name=(val) 
    @alias_table_name = val 
  end 

  def alias_table_name 
    @alias_table_name ||= "#{self.table_name}_alias" 
  end   

  def alias_source
    @alias_klass ||= Class.new(self).tap do |k|
      k.all.source.left.table_alias = alias_table_name
    end  
  end 
end 

Then usage as:

class User < ApplicationRecord 
  extend ARTableAlias 

  alias_table_name = :toto
end 

User.alias_source.where(first_name = 'Ruur') 
#=> SELECT `toto`.* FROM `users` `toto` WHERE `toto`.`first_name` = 'Ruur'
User.where(first_name = 'Ruur') 
#=> SELECT `users`.* FROM `users` WHERE `users`.`first_name` = 'Ruur'
User.alias_source === User.alias_source
#=> true