[Rails] difference between build_ and .build

hi im new in Ruby on rails and currently working with a reservation
system can anyone explain what is the difference between
current_package = build_reservation_package(:package_id => package_id)
abd
current_package =reservation_package.build(:package_id => package_id)
because i tried using the first one but no good but when i try the
second it worked i think both are the same please correct me if im
wrong thank

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