What is best way to define AR scope for this?

rubyonrails-talk
#1

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.

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

#2

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

#3

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.

#4

I see what you mean. Very very helpful. Thanks both so much for the
timely reply!

I think I will go with what you are suggesting and use the boolean flag
on the join table.

Still open to other answers if there are any for sake of curiosity, and
I will add comment if I come across anything else, but I think I will go
with what you said. Much simpler that way.

Thanks again.