Rails string as a foreign key - ruby-on-rails

Question

I have a relation between User and Course (typical enrollment data). A User has_many Course and vice-versa (typical JOIN table scenario).
I am attempting to migrate my previous has_and_belongs_to_many relationship between these two models to a has_many :through relationship. My files currently look like:
class User < ActiveRecord::Base
has_and_belongs_to_many :courses
end
and
class Course < ActiveRecord::Base
has_and_belongs_to_many :users
end
and the table name that joins the two models is courses_users.
I now need to migrate this relationship to the has_many :through association, and also make the column type of user_id a string, as I want to use the g_number (string) attribute of User as the foreign key. Note: I don't care about the performance difference between int and varchar/string.
The short and simple problem is that I need users.g_number to reference enrollments.user_id as a foreign key, and both are strings.
My attempt at a migration and model rework is this:
class User < ActiveRecord::Base
has_many :enrollment
has_many :courses, :through => :enrollment
end
and
class Course < ActiveRecord::Base
has_many :enrollment
has_many :users, :through => :enrollment
end
lastly
class Enrollment < ActiveRecord::Base
belongs_to :course
belongs_to :user
end
then the migration
class ChangeUserIdJoin < ActiveRecord::Migration
def self.up
rename_table :courses_users, :enrollments
end
def self.down
rename_table :enrollments, :courses_users
end
end
Everything works fine here. I can do queries like User.courses and Course.users. But now I want to change the type of the user_id column in the join table to a string so that I can store the g_number (string attribute on User) and join on that instead of the serial id column of User.
When I attempt to change the user_id column type to string in the migration:
class ChangeUserIdJoin < ActiveRecord::Migration
def self.up
change_column :courses_users, :user_id, :string
rename_table :courses_users, :enrollments
end
def self.down
rename_table :enrollments, :courses_users
change_column :courses_users, :user_id, :integer
end
end
the queries Course.users and User.courses start failing (below from Rails console). User.courses returns an empty array (whereas before there are multiple Course objects), and Course.users throws an exception because of mismatched column types (which obviously makes sense):
u = User.take
User Load (0.9ms) SELECT "users".* FROM "users" LIMIT 1
=> #<User id: 1, username: "director", g_number: "g00000000", password_digest: "$2a$10$dvcOd3rHfbcR1Rn/D6VhsOokj4XiIkQbHxXLYjy5s4f...", created_at: "2016-01-06 01:36:00", updated_at: "2016-01-06 01:36:00", first_name: "Director", last_name: "", role: 0, registered: true>
2.1.5 :002 > u.courses
Course Load (0.9ms) SELECT "courses".* FROM "courses" INNER JOIN "enrollments ON "courses"."id" = "enrollments"."course_id" WHERE "enrollments"."user_id" = $1 [["user_id", 1]]
=> #<ActiveRecord::Associations::CollectionProxy []>
2.1.5 :003 > c = Course.take
Course Load (0.7ms) SELECT "courses".* FROM "courses" LIMIT 1
=> #<Course id: 12754, year: 2015, semester: 0, department: 7, course: 101, section: 1, name: "SPA 101 01 - Elementary Spanish I">
2.1.5 :004 > c.users
PG::UndefinedFunction: ERROR: operator does not exist: integer = character varying
LINE 1: ... "users" INNER JOIN "enrollments" ON "users"."id" = "enrollm...
I need to be able to join on enrollments.user_id = users.g_number. What do I need to do in order to change the user_id column to a string type in the Enrollment model/table, and still be able to do Active Record queries like User.courses and Course.users?

Answer 1

Try by specifying the foreign and primary keys in enrollments model, like this
belongs_to :user foreign_key: :user_id, primary_key: :g_number