can activerecord association with the functional foreign key

As we know the model can associate other model through the foreign key which is assigned or with an “_id” suffix by default. But can we define functional foreign key by ourselves ?


  class Condition < ActiveRecord::Base
belongs_to :city_info, foreign_key: "substring_index(the_city,',',1)" end
class CityInfo < ActiveRecord::Base
>> Condition.includes(:city_info).references(:city_info).last
>> Mysql2::Error: Unknown column 'conditions.substring_index(the_city,',',1)' in 'on clause': SELECT ..... ......`conditions`.`substring_index(the_city,',',1)` ORDER BY `conditions`.`id` DESC LIMIT 1

It’s wrong. How can we achieve this.

I want to retrieve many conditions object, each associates with its first city_info by ActiveRecord with generating a single sql (not n+1

) like this:

select * from conditions left outer join city_info on convert(substring_index(conditions.the_city,',',1), unsigned integer) =