Uninitialized constant error in Ruby class

I have these two classes in RubyMine:

book.rb:

 class Book
   def initialize(name,author)
   end
 end

test.rb:

require 'book'
class teste
   harry_potter = Book.new("Harry Potter", "JK")
end

When I run test.rb, I get this error:

C:/Users/DESKTOP/RubymineProjects/learning/test.rb:3:in `<class:Test>': uninitialized constant Test::Book (NameError)
from C:/Users/DESKTOP/RubymineProjects/learning/test.rb:1:in `<top (required)>'
from -e:1:in `load'
from -e:1:in `<main>'

In a Rails app this error can also be caused by renaming the class without renaming the file to match, which was my issue when I found this error:

book.rb

class Book
  def initialize(name, author)
  end
end

book_test.rb

class BookTest
  harry_potter = Book.new("Harry Potter", "JK")
end