Rails core - Mar 2014 - 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 ?

eg:

class Condition < ActiveRecord::Base
  belongs_to :city_info, foreign_key:
"substring_index(the_city,',',1)"
end

class CityInfo < ActiveRecord::Base
end
>> 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) =
city_info.id

ref: https://github.com/rails/rails/issues/14481

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