Adds uniqueness constraint on paper-SNP connection. Replaces rake task by SQL command.

app/models/snp_reference.rb

@@ -3,4 +3,5 @@ class SnpReference < ActiveRecord::Base
   belongs_to :snp
   belongs_to :paper, polymorphic: true
   #validates_presence_of :snp, :paper
+  validates_uniqueness_of :snp_id, scope: [:paper_id, :paper_type]
 end

lib/tasks/make_paper_snps_unique.rake

@@ -1,9 +1,9 @@
 namespace :papers do
   task :make_linked_snps_unique => :environment do
-    %w(MendeleyPaper SnpediaPaper PlosPaper).each do |source|
-      source.constantize.find_each do |s|
-        s.update(snps: s.snps.uniq)
-      end
-    end
+    ActiveRecord::Base.connection.execute(<<-SQL)
+      SELECT DISTINCT * INTO new_table FROM snp_references;
+      DROP TABLE snp_references;
+      ALTER TABLE new_table RENAME TO snp_references;
+    SQL
   end
 end