Rails model with foreign_key and link table

Question

I am trying to create a model for a ruby on rails project that builds relationships between different words. Think of it as a dictionary where the "Links" between two words shows that they can be used synonymously. My DB looks something like this:

Words
----
id

Links
-----
id
word1_id
word2_id

How do I create a relationship between two words, using the link-table. I've tried to create the model but was not sure how to get the link-table into play:

class Word < ActiveRecord::Base
  has_many :synonyms, :class_name => 'Word', :foreign_key => 'word1_id'
end

Answer 1

Hmm, this is a tricky one. That is because synonyms can be from either the word1 id or the word2 id or both.

Anyway, when using a Model for the link table, you must use the :through option on the Models that use the Link Table

class Word < ActiveRecord::Base
  has_many :links1, :class_name => 'Link', :foreign_key => 'word1_id'
  has_many :synonyms1, :through => :links1, :source => :word
  has_many :links2, :class_name => 'Link', :foreign_key => 'word2_id'
  has_many :synonyms2, :through => :links2, :source => :word
end

That should do it, but now you must check two places to get all the synonyms. I would add a method that joined these, inside class Word.

def synonyms
  return synonyms1 || synonyms2
end

||ing the results together will join the arrays and eliminate duplicates between them.

*This code is untested.

Answer 2

I'd view it from a different angle; since all the words are synonymous, you shouldn't promote any one of them to be the "best". Try something like this:

class Concept < ActiveRecord::Base
  has_many :words
end

class Word < ActiveRecord::Base
  belongs_to :concept

  validates_presence_of :text
  validates_uniqueness_of :text, :scope => :concept_id

  # A sophisticated association would be better than this.
  def synonyms
    concept.words - [self]
  end
end

Now you can do

word = Word.find_by_text("epiphany")
word.synonyms

Answer 3

In general, if your association has suffixes such as 1 and 2, it's not set up properly. Try this for the Word model:

class Word < ActiveRecord::Base
  has_many :links, :dependent => :destroy
  has_many :synonyms, :through => :links
end

Link model:

class Link < ActiveRecord::Base
  belongs_to :word
  belongs_to :synonym, :class_name => 'Word'

  # Creates the complementary link automatically - this means all synonymous
  # relationships are represented in @word.synonyms
  def after_save_on_create
    if find_complement.nil?
      Link.new(:word => synonym, :synonym => word).save
    end
  end

  # Deletes the complementary link automatically.
  def after_destroy
    if complement = find_complement
      complement.destroy
    end
  end

  protected

  def find_complement
    Link.find(:first, :conditions => 
      ["word_id = ? and synonym_id = ?", synonym.id, word.id])
  end
end

Tables:

Words
----
id

Links
-----
id
word_id
synonym_id

Answer 4

Word model:

class Word < ActiveRecord::Base
  has_many :links, :dependent => :destroy
  has_many :synonyms, :through => :links

  def link_to(word)
    synonyms << word
    word.synonyms << self
  end
end

Setting :dependent => :destroy on the has_many :links will remove all the links associated with that word before destroying the word record.

Link Model:

class Link < ActiveRecord::Base
  belongs_to :word
  belongs_to :synonym, :class_name => "Word"
end

Assuming you're using the latest Rails, you won't have to specify the foreign key for the belongs_to :synonym. If I recall correctly, this was introduced as a standard in Rails 2.

Word table:

name

Link table:

word_id
synonym_id

To link an existing word as a synonym to another word:

word = Word.find_by_name("feline")
word.link_to(Word.find_by_name("cat"))

To create a new word as a synonym to another word:

word = Word.find_by_name("canine")
word.link_to(Word.create(:name => "dog"))