Use use fk_name for _id column

app/models/mixins/purging_mixin.rb

@@ -111,12 +111,13 @@ def purge_orphans(fk_name, window)
       # For now, this only supports polymorphic references
       # We don't currently have a situation where a table with a regular reference needs to be purged
       polymorphic_type_column = "#{fk_name}_type"
+      polymorphic_id_column   = connection.quote_column_name("#{fk_name}_id")
       total = 0
 
       polymorphic_classes(polymorphic_type_column).each do |klass|
         resource_table = connection.quote_table_name(klass.table_name)
 
-        scope = joins("LEFT OUTER JOIN #{resource_table} ON #{table_name}.resource_id = #{resource_table}.id")
+        scope = joins("LEFT OUTER JOIN #{resource_table} ON #{table_name}.#{polymorphic_id_column} = #{resource_table}.id")
                 .where(resource_table => {:id => nil})
                 .where("#{table_name}.#{connection.quote_column_name(polymorphic_type_column)} = #{connection.quote(klass.name)}")
         total += purge_in_batches(scope, window)