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    copied!<p>Here's a solution using a self-referential relationship using single table inheritance. Use these commands to create the app:</p> <pre><code>$ rails myproject $ cd myproject $ script/generate model publication type:string name:string $ script/generate model citation publication_id:integer reference_id:integer </code></pre> <p>The setup the relationships this way:</p> <pre><code>class Publication &lt; ActiveRecord::Base has_many :citations has_many :cited_publications, :through =&gt; :citations, :source =&gt; :reference has_many :references, :foreign_key =&gt; "reference_id", :class_name =&gt; "Citation" has_many :refered_publications, :through =&gt; :references, :source =&gt; :publication end class Citation &lt; ActiveRecord::Base belongs_to :publication belongs_to :reference, :class_name =&gt; "Publication" end class Article &lt; Publication end class Book &lt; Publication end class Chapter &lt; Publication end </code></pre> <p>Now we can create the DB and try it out from the console:</p> <pre><code>$ rake db:migrate $ script/console Loading development environment (Rails 2.2.2) &gt;&gt; a = Article.create!(:name =&gt; "Article") =&gt; #&lt;Article id: 1, ...&gt; &gt;&gt; b = Book.create!(:name =&gt; "Book") =&gt; #&lt;Book id: 2, ...&gt; &gt;&gt; a.citations.create(:reference =&gt; b) =&gt; #&lt;Citation id: 1, publication_id: 1, reference_id: 2, created_at: "2009-02-15 14:13:15", updated_at: "2009-02-15 14:13:15"&gt; &gt;&gt; a.citations =&gt; [#&lt;Citation id: 1, ...&gt;] &gt;&gt; a.references =&gt; [] &gt;&gt; b.citations =&gt; [] &gt;&gt; b.references =&gt; [#&lt;Citation id: 1, publication_id: 1, reference_id: 2, created_at: "2009-02-15 14:13:15", updated_at: "2009-02-15 14:13:15"&gt;] &gt;&gt; a.cited_publications =&gt; [#&lt;Book id: 2, type: "Book", name: "Book", created_at: "2009-02-15 14:11:00", updated_at: "2009-02-15 14:11:00"&gt;] &gt;&gt; a.refered_publications =&gt; [] &gt;&gt; b.cited_publications =&gt; [] &gt;&gt; b.refered_publications =&gt; [#&lt;Article id: 1, type: "Article", name: "Article", created_at: "2009-02-15 14:10:51", updated_at: "2009-02-15 14:10:51"&gt;] </code></pre>
 

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