给源表起别名

Question

有没有办法在单个作用域的上下文中为源表设置别名？

我试过了：

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

问题是模型类为所有后续查询保留别名：

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

编辑

我把这个方法加到ApplicationRecord

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

现在，我可以做：

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

Answer 1

我将此方法添加到 ApplicationRecord

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

现在，我可以做：

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

Answer 2

由于创建从ActiveRecord::Base 继承的匿名类会导致内存膨胀，我不确定我是否会推荐它（参见此处：https://github.com/rails/rails/issues/31395）。

也许更好的实现是以块形式执行，而不是例如将别名表生成给块，然后再将其设置回来？

例如

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

用法

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`

警告：这还没有在此处显示的范围之外进行测试，我不建议在没有对边缘外壳和其他问题进行广泛测试的生产环境中进行此操作

更新以解决所需的实施

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 

然后用法为：

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