Ruby on Rails

Yes, I'd agree with what Colin wrote.

Generally when you try to have one thing join via a join table ( has many through) and the same set of data also join via a belongs_to you get headaches because you're going a bit against the grain of AR.

You could still implement top-level methods to do what you need, but treat all tags as has_many :through in the AR relationship. You could even put a boolean flag into that join table (tag_listings) to indicate "is_primary"



class TagListings
scope :primary, where(:primary => true)
validates_uniqueness :primary, :scope => :article_id # this makes sure there is only 1 primary tag for any given article
end


then from your Article class
@article.tag_listings.primary would get you the primary tag


Then you kind of eliminate the whole dual-model problem

(in my head the code above works, although I haven not tried)

Just giving you this as perspective as an alternative implementation.



On Sep 4, 2014, at 2:30 PM, Colin Law <clanlaw@gmail.com> wrote:

> On 4 September 2014 18:45, Michael Bostler <lists@ruby-forum.com> wrote:
>> Hi all,
>>
>>
>> Say I have an article class that has one main tag, but can also be
>> associated with multiple secondary tags. Something like schema listed
>> below. How can I implement the scope method below?
>>
>>
>> Basically I want one method where I can return ALL articles related to a
>> tag, regardless of whether it is a primary tag relationship or secondary
>> tag relationship.
>
> You might consider a different schema to that you have shown below, so
> that article has_many tags through tag_listings, and in tag_listings
> have a flag to indicate a primary tag (or not). Then you get the
> primary tag by selecting the tag with that flag set. Then the problem
> you have asked about becomes not a problem (which is the best sort).
> Still not ideal I know, as you have to guard against the possibility
> of inadvertently ending up with multiple primary tags. Perhaps there
> is a better solution.
>
> Colin
>
>>
>>
>> Many Thanks!
>>
>>
>> class Article < ActiveRecord::Base
>> belongs_to :primary_tag,
>> :class_name => "Tag"
>>
>> has_many :secondary_tag_listings
>>
>> has_many :secondary_tags,
>> :through => :secondary_tag_listings,
>> :class_name => "Tag"
>>
>> scope :all_with_tag, lambda { |tag_name|
>> # TODO: how to find all with either primary OR secondary tags
>> # as tag_name?
>> }
>> end
>>
>>
>>
>>
>> class SecondaryTagListings < ActiveRecord::Base
>> belongs_to :article
>> belongs_to :tag
>> end
>>
>>
>>
>>
>> class Tag < ActiveRecord::Base
>> has_many :primary_articles,
>> :class_name => "Article"
>>
>> has_many :secondary_tag_listings
>>
>> has_many :secondary_articles,
>> :through => :secondary_tag_listings,
>> :class_name => "Article"
>> end
>>
>> --
>> Posted via http://www.ruby-forum.com/.
>>
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