In controller specs, since you are not fetching data from the DB, you don't have to save the record in the DB. 

Using the DB slows down the specs.

FactoryGirl has a method 'build' which just initializes the object but doesn't save it to the DB.

For example, here you can write: @comment1 = FactoryGirl.build(:comment, :user => @user, :tweet => @tweet)

You can learn more about this and mocking models in the RSpec book.