has_and_belongs_to_many, avoiding dupes in the join table

Question

I have a pretty simple HABTM set of models

class Tag < ActiveRecord::Base 
   has_and_belongs_to_many :posts
end 

class Post < ActiveRecord::Base 
           


        

Answer 1

I would prefer to adjust the model and create the classes this way:

class Tag < ActiveRecord::Base 
   has_many :taggings
   has_many :posts, :through => :taggings
end 

class Post < ActiveRecord::Base 
   has_many :taggings
   has_many :tags, :through => :taggings
end

class Tagging < ActiveRecord::Base 
   belongs_to :tag
   belongs_to :post
end

Then I would wrap the creation in logic so that Tag models were reused if it existed already. I'd probably even put a unique constraint on the tag name to enforce it. That makes it more efficient to search either way since you can just use the indexes on the join table (to find all posts for a particular tag, and all tags for a particular post).

The only catch is that you can't allow renaming of tags since changing the tag name would affect all uses of that tag. Make the user delete the tag and create a new one instead.

Answer 2

I worked around this by creating a before_save filter that fixes stuff up.

class Post < ActiveRecord::Base 
   has_and_belongs_to_many :tags
   before_save :fix_tags

   def tag_list= (tag_list) 
      self.tags.clear 
      tag_list.strip.split(' ').each do 
        self.tags.build(:name => tag) 
      end
   end  

    def fix_tags
      if self.tags.loaded?
        new_tags = [] 
        self.tags.each do |tag|
          if existing = Tag.find_by_name(tag.name) 
            new_tags << existing
          else 
            new_tags << tag
          end   
        end

        self.tags = new_tags 
      end
    end

end

It could be slightly optimised to work in batches with the tags, also it may need some slightly better transactional support.

Answer 3

This is really old but I thought I'd share my way of doing this.

class Tag < ActiveRecord::Base 
    has_and_belongs_to_many :posts
end 

class Post < ActiveRecord::Base 
    has_and_belongs_to_many :tags
end

In the code where I need to add tags to a post, I do something like:

new_tag = Tag.find_by(name: 'cool')
post.tag_ids = (post.tag_ids + [new_tag.id]).uniq

This has the effect of automatically adding/removing tags as necessary or doing nothing if that's the case.

Answer 4

In Rails4:

class Post < ActiveRecord::Base 
  has_and_belongs_to_many :tags, -> { uniq }

(beware, the -> { uniq } must be directly after the relation name, before other params)

Rails documentation

Answer 5

You should add an index on the tag :name property and then use the find_or_create method in the Tags#create method

docs

Answer 6

Just add a check in your controller before adding the record. If it does, do nothing, if it doesn't, add a new one:

u = current_user
a = @article
if u.articles.exists?(a)

else
  u.articles << a
end

More: "4.4.1.14 collection.exists?(...)" http://edgeguides.rubyonrails.org/association_basics.html#scopes-for-has-and-belongs-to-many