warning: constant ::Fixnum is deprecated When generating new model

This warnings appear because you are using ruby 2.4.0.

This version introduced this change: Unify Fixnum and Bignum into Integer

See here for the announcement: https://www.ruby-lang.org/en/news/2016/12/25/ruby-2-4-0-released/

The warnings come from the activesupport gem which is part of rails and will be fixed in an upcoming release.

For now you can just ignore those warnings.

Update: Rails 5.0.2 has been released, which gets rid of the warnings.

I fixed mine by updating rails

bundle update rails

I assume you're using Rails 5? Check out this link (towards the bottom). Looks like these warnings will go away with release #27458.