Ruby class with array attribute - ruby

Question

I have a non-database-backed class in Ruby:
class User
attr_accessor :countries
end
I want countries to simply be an array of ISO country codes (US, GB, CA, AU, etc) and I don't want to build a separate model to hold each. Is there a magic way to make Ruby understand that :countries is an array and treat it accordingly, or do I need to write the countries and countries= methods?
I tried just setting the countries array with user.countries = ['US'], and I'm getting a NoMethodError.

Answer 1

The type of a variable doesn't matter in Ruby.
attr_accessor just creates getter and setter methods that set and return instance variables; #countries in this case. You can set the instance variable to your array, or use the setter:
class User
attr_accessor :countries
def initialize
#countries = %w[Foo Bar Baz]
# Or...
self.countries = %w[Foo Bar Baz]
end
end
> puts User.new.countries
=> ["Foo", "Bar", "Baz"]
Personally I prefer using the instance variable instead of self.xxx; it's too easy to forget the self. bit and you end up setting a local variable, leaving the instance variable nil. I also think it's ugly.
If the countries won't be changing between instances, why not a constant?
Edit/Clarification
Tadman's point is well-taken, e.g., this diatribe on state. The circumtances under which I don't care about that are limited to small, self-controlled, stand-alone classes. There are inherent risks in making those assumptions, the level of those risks is project-dependent.

Answer 2

Looks like countries should be a constant:
class User
COUNTRIES = %w(
AF AX AL DZ AS AD AO AI AQ AG AR AM AW AU AT AZ BS BH BD BB BY BE BZ BJ BM
BT BO BQ BA BW BV BR IO BN BG BF BI KH CM CA CV KY CF TD CL CN CX CC CO KM
CG CD CK CR CI HR CU CW CY CZ DK DJ DM DO EC EG SV GQ ER EE ET FK FO FJ FI
FR GF PF TF GA GM GE DE GH GI GR GL GD GP GU GT GG GN GW GY HT HM VA HN HK
HU IS IN ID IR IQ IE IM IL IT JM JP JE JO KZ KE KI KP KR KW KG LA LV LB LS
LR LY LI LT LU MO MK MG MW MY MV ML MT MH MQ MR MU YT MX FM MD MC MN ME MS
MA MZ MM NA NR NP NL NC NZ NI NE NG NU NF MP NO OM PK PW PS PA PG PY PE PH
PN PL PT PR QA RE RO RU RW BL SH KN LC MF PM VC WS SM ST SA SN RS SC SL SG
SX SK SI SB SO ZA GS SS ES LK SD SR SJ SZ SE CH SY TW TJ TZ TH TL TG TK TO
TT TN TR TM TC TV UG UA AE GB US UM UY UZ VU VE VN VG VI WF EH YE ZM ZW
).freeze
end
User::COUNTRIES.include? "US" #=> true
freeze prevents modifications:
User::COUNTRIES.delete "US" #=> RuntimeError: can't modify frozen Array
Update
The problem here is that your countries array has to be persisted somehow. You are mentioning has_many so Rails seems to be involved. You can use ActiveRecord's serialize method:
class User < ActiveRecord::Base
serialize :countries
end
This will save the countries attribute to the database as an object and retrieve it as such:
u = User.new
u.countries = ["US", "CA"]
u.save
u = User.last
u.countries
#=> ["US", "CA"]
It's converted to and from YAML internally, so the users table looks like:
mysql> SELECT * FROM users;
+----+-------------------+---------------------+---------------------+
| id | countries | created_at | updated_at |
+----+-------------------+---------------------+---------------------+
| 1 | ---\n- US\n- CA\n | 2013-09-24 18:24:03 | 2013-09-24 18:24:03 |
+----+-------------------+---------------------+---------------------+
1 row in set (0,00 sec)