Rails - Custom Validation For Having A Single Value Once - ruby-on-rails

Question

so I have these two models:
class Tag < ActiveRecord::Base
has_many :event_tags
attr_accessible :tag_id, :tag_type, :value
end
class EventTag < ActiveRecord::Base
belongs_to :tag
attr_accessible :tag_id, :event_id, :region
end
and this table for Tags:
**tag_id** **tag_type** **value**
1 "funLevel" "Boring..."
2 "funLevel" "A Little"
3 "funLevel" "Hellz ya"
4 "generic" "Needs less clowns"
5 "generic" "Lazer Tag"
...
What I would like to do is write a custom validation where it checks to see:
Each event_id has only one tag_type of "funLevel" attached to it, but can have more than one "generic" tags
For example:
t1 = EventTag.new(:tag_id => 1, :event_id =>777, :region => 'US')
t1.save # success
t2 = EventTag.new(:tag_id => 2, :event_id =>777, :region => 'US')
t2.save # failure
# because (event_id: 777) already has a tag_type of
# "funLevel" associated with it
t3 = EventTag.new(:tag_id => 4, :event_id =>777, :region => 'US')
t3.save # success, because as (tag_id:4) is not "funLevel" type
I have come up with one ugly solution:
def cannot_have_multiple_funLevel_tag
list_of_tag_ids = EventTag.where("event_id = ?", event_id).pluck(:tag_id)
if(Tag.where("tag_id in ?", list_of_tag_ids).pluck(:tag_type).include? "funLevel")
errors.add(:tag_id, "Already has a Fun Level Tag!")
end
Being new to rails, is there a more better/more elegant/more inexpensive way?

Answer 1

The way you have your data structured means that the inbuilt Rails validations are probably not going to be a heap of help to you. If the funLevel attribute was directly accessible by the EventTag class, you could just use something like:
# event_tag.rb
validate :tag_type, uniqueness: { scope: :event_id },
if: Proc.new { |tag| tag.tag_type == "funLevel" }
(unfortunately, from a quick test you don't seem to be able to validate the uniqueness of a virtual attribute.)
Without that, you're probably stuck using a custom validation. The obvious improvement to the custom validation you have (given it looks like you want to have the validation on EventTag) would be to not run the validation unless that EventTag is a funLevel tag:
def cannot_have_multiple_funLevel_tag
return unless self.tag.tag_type == "funLevel"
...
end