Help me the error after replace sqlite3 by using MongoDb and MongoId

I want use MongoDB same as database of Project.
I delete gem "sqlite3" and copied 2 gem:
gem "mongoid", "~> 3.0.0"
gem 'bson_ext'
then had error :
in `block in replace_gem': Please install the sqlite3 adapter: `gem
install activerecord-sqlite3-adapter` (sqlite3 is not part of the
bundle. Add it to Gemfile.) (LoadError)
and if I copy gem sqlite3 the program ran with MongoDb same as database
Plase instruct for me this problem and how to do if I want only MongoDB
same as Database.
Thanks...

It isn’t enough just to change the gems, you also have to alter the database configuration to be found at config/database.yml. Please check Mongo manual for further details.

Thanks Norbert Melzer
I read railscast 238(mongoid)
If I use command line:

rails g mongoid:config
then in my project have 2 DB :sqlite3+mongoDB
And I see data after insert then save in MongoDB
If I using both sqlite3+mongoDB above ,please tell me about my program
have error with process data after?
thanks.