Extract duplicated query

This includes a bit of a simplification which assumes that there aren't
any orphan taggings for a user.

app/models/stats/user_tags_query.rb

@@ -0,0 +1,26 @@
+module Stats
+  class UserTagsQuery
+
+    attr_reader :user
+    def initialize(user)
+      @user = user
+    end
+
+    def result
+      user.todos.find_by_sql([sql, user.id])
+    end
+
+    private
+
+    def sql
+      <<-SQL
+        SELECT tags.id as id
+        FROM tags, taggings, todos
+        WHERE tags.id = taggings.tag_id
+        AND taggings.taggable_id = todos.id
+        AND todos.user_id = ?
+      SQL
+    end
+
+  end
+end