Ruby on Rails

hi everyone..
i install rails and all the things which is need for.
i create a new project
i create datbase's and set the database.yml file

but when i go to browser and type this=> localhost:3000. i get this
error=>

Gem::Loaderror

Specified 'postgresql' for database adapter, but the gem is not loaded.
Add `gem 'pg'` to your Gemfile (and ensure its version is at the minimum
required by ActiveRecord).

for fixing it i uninstall all the gems and reinstall all of them.
but again i have this error.

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