Rails: How to call one-to-one relationship in rails - ruby-on-rails

Question

I'm new to rails and I want to know how to fetch a one-to-one relationship. I want to fetch users city. In my postgresql database I have:
cities Table:
city:varchar
zipcode: integer
users Table
name:varchar
city_id:int
and in city and user model I have:
class City < ActiveRecord::Base
belongs_to :user
end
class User < ActiveRecord::Base
has_one :city
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
end
I tried the following in my search controller but didnt work, when logged in:
current_user.city
I get the following error
Processing by SearchController#index as HTML
Parameters: {"utf8"=>"✓", "q"=>"", "criteria"=>"1", "commit"=>"Search"}
User Load (1.1ms) SELECT "users".* FROM "users" WHERE "users"."id" = 6 ORDER BY "users"."id" ASC LIMIT 1
PG::UndefinedColumn: ERROR: column cities.user_id does not exist
LINE 1: SELECT "cities".* FROM "cities" WHERE "cities"."user_id" =...
^
: SELECT "cities".* FROM "cities" WHERE "cities"."user_id" = $1 LIMIT 1
Completed 500 Internal Server Error in 11ms
ActiveRecord::StatementInvalid (PG::UndefinedColumn: ERROR: column cities.user_id does not exist
LINE 1: SELECT "cities".* FROM "cities" WHERE "cities"."user_id" =...
^
: SELECT "cities".* FROM "cities" WHERE "cities"."user_id" = $1 LIMIT 1):
why am I suppose to add a user_id column to cities table, when I have cities foreign key in users table? I dont want to add user_id into cities table.

Answer 1

You can use has_one :through association with join table. Some example for you below.
user model:
class User < ActiveRecord::Base
has_one :city, through: :user_city
has_one :user_city
end
city model:
class City < ActiveRecord::Base
belongs_to :user
end
user city join model:
class UserCity < ActiveRecord::Base
belongs_to :city
belongs_to :user
end
migration for join tables:
class JoinUserCity < ActiveRecord::Migration
def change
create_table :user_cities do |t|
t.integer :user_id
t.integer :city_id
end
end
end
Test in rails console:
=> u = User.create
(0.1ms) begin transaction
SQL (0.5ms) INSERT INTO "users" ("created_at", "updated_at") VALUES (?, ?) [["created_at", "2014-12-07 15:47:14.595728"], ["updated_at", "2014-12-07 15:47:14.595728"]]
(3.3ms) commit transaction
=> #<User id: 4, created_at: "2014-12-07 15:47:14", updated_at: "2014-12-07 15:47:14">
=> u.city
City Load (0.2ms) SELECT "cities".* FROM "cities" INNER JOIN "user_cities" ON "cities"."id" = "user_cities"."city_id" WHERE "user_cities"."user_id" = ? LIMIT 1 [["user_id", 4]]
=> nil
=> c = City.create
(0.1ms) begin transaction
SQL (0.5ms) INSERT INTO "cities" ("created_at", "updated_at") VALUES (?, ?) [["created_at", "2014-12-07 15:47:24.535039"], ["updated_at", "2014-12-07 15:47:24.535039"]]
(3.3ms) commit transaction
=> #<City id: 1, created_at: "2014-12-07 15:47:24", updated_at: "2014-12-07 15:47:24">
irb(main):004:0> u.city = c
UserCity Load (0.3ms) SELECT "user_cities".* FROM "user_cities" WHERE "user_cities"."user_id" = ? LIMIT 1 [["user_id", 4]]
(0.1ms) begin transaction
SQL (0.4ms) INSERT INTO "user_cities" ("city_id", "user_id") VALUES (?, ?) [["city_id", 1], ["user_id", 4]]
(1.0ms) commit transaction
=> #<City id: 1, created_at: "2014-12-07 15:47:24", updated_at: "2014-12-07 15:47:24">
irb(main):005:0> u.save
(0.1ms) begin transaction
(0.1ms) commit transaction
=> true
=> u = User.last
User Load (0.3ms) SELECT "users".* FROM "users" ORDER BY "users"."id" DESC LIMIT 1
=> #<User id: 4, created_at: "2014-12-07 15:47:14", updated_at: "2014-12-07 15:47:14">
=> u.city
City Load (0.2ms) SELECT "cities".* FROM "cities" INNER JOIN "user_cities" ON "cities"."id" = "user_cities"."city_id" WHERE "user_cities"."user_id" = ? LIMIT 1 [["user_id", 4]]
=> #<City id: 1, created_at: "2014-12-07 15:47:24", updated_at: "2014-12-07 15:47:24">

Answer 2

take a look at the document of has_one and belogns_to,
belongs_to(name, options = {})
Specifies a one-to-one association with another class. This method should only be used if this class
contains the foreign key. If the other class contains the foreign key, then you should use has_one
instead.
as the user table has the foreign key, you should change your model definition like this
class City < ActiveRecord::Base
has_one :user
end
class User < ActiveRecord::Base
belongs_to :city
end