Raise error when including (instead of extending) PgSearch

lib/pg_search.rb

@@ -55,6 +55,10 @@ def extended mod
       mod.send :include, Compatibility::ActiveRecord3
     end
 
+    def included _
+      raise 'extend PgSearch instead of including it'
+    end
+
     def multisearch(*args)
       PgSearch::Document.search(*args)
     end

spec/lib/pg_search_spec.rb

@@ -0,0 +1,9 @@
+require 'spec_helper'
+
+describe PgSearch do
+  it "raises an error when included" do
+    expect do
+      Module.new { include PgSearch }
+    end.to raise_error 'extend PgSearch instead of including it'
+  end
+end