如何获得HABTM协会AREL表？ [英] How to get the arel table of a habtm association?

问题描述

我有两个的ActiveRecord 机型，具有HABTM关联。

I have two ActiveRecord models which have a HABTM association.

我想写一个范围使用阿雷尔得到孤立记录。

I want to write a scope to get the orphan records using Arel.

我的问题是，我无法找到一个检索关联的 arel_table 方法。由于关系HABTM，没有模型来调用 arel_table 上。

My problem is that I couldn't find a method to retrieve the arel_table of the association. Since the relation is HABTM, there is no model to call arel_table on.

我现在以下（其中工程），但我做的连接表的名称的新AREL表（通过使用 reflect_on_association 方法检索）。

I have the following now (which works), but I make a new arel table with the name of the join table (retrieved by using the reflect_on_association method).

scope :orphans, lambda {
    teachers = arel_table
    join_table = Arel::Table.new(reflect_on_association(:groups).options[:join_table])

    join_table_condition = join_table.project(join_table[:teacher_id])
    where(teachers[:id].not_in(join_table_condition))
}

这生成的SQL语句：

SELECT `teachers`.* 
FROM `teachers`    
WHERE (`teachers`.`id` NOT IN (SELECT `groups_teachers`.`teacher_id`  
                               FROM `groups_teachers` ))

那么，有没有更好的方法来检索的 arel_table ，而不是做一个新的？

So is there any better way to retrieve the arel_table instead of making a new one?

推荐答案

不幸的是，我相信你的解决方案是pretty的许多干净的存在，现在，是的，其实，什么样的关联本身内部确实实例化时：

Unfortunately, I believe your solution is pretty much the cleanest there is right now and is, in fact, what the association itself does internally when instantiated:

<一个href="https://github.com/rails/rails/blob/46492949b8c09f99db78b9f7a02d039e7bc6a702/activerecord/lib/active_record/associations/has_and_belongs_to_many_association.rb#L7" rel="nofollow">https://github.com/rails/rails/blob/46492949b8c09f99db78b9f7a02d039e7bc6a702/activerecord/lib/active_record/associations/has_and_belongs_to_many_association.rb#L7

我相信他们使用VS reflection.options的reflection.join_table [：join_table]仅在主，现在虽然

I believe the reflection.join_table they use vs reflection.options[:join_table] is only in master right now though.

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