BSON::InvalidDocument: Cannot serialize an object of class ActiveRecord::ConnectionAdapters::SQLite

[googya]

BSON::InvalidDocument: Cannot serialize an object of class ActiveRecord::ConnectionAdapters::SQLiteC│~
olumn into BSON.: INSERT INTO "users" ("age", "created_at", "name", "updated_at") VALUES (?, ?, ?, ?

In the chapter about "Storing notifications in the Database", I met a error given above.

no working of enginex under rails 3.2, so I use

• rails plugin new SqlMetrics, all tests are passed, and
• when executing "bundle exec rails generate scaffold User name:string age:integer" under test/dummy ,
• creating a user object and save it ,
• then the error is thrown.