How to get isl table from habtm association?

Question

I have two models ActiveRecordthat have a HABTM association.

I want to write scopeto get orphaned entries using Arel.

My problem is that I could not find a method to extract arel_tablefrom the association. Since the relationship is HABTM, there is no model to call arel_tableon.

Now I have the following (which works), but I create a new isl table with the name of the join table (retrieved using the method reflect_on_association).

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))
}

As a result, the following SQL is created:

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

So, is there a better way to get arel_tableinstead of creating a new one?

Answer 1

, , , , , :

https://github.com/rails/rails/blob/46492949b8c09f99db78b9f7a02d039e7bc6a702/activerecord/lib/active_record/associations/has_and_belongs_to_many_association.rb#L7

, .join_table vs reflection.options [: join_table] master.

Answer 2

, , , , , :

Arel::Table.new(:groups_teachers)

Arel habtm. , .