ruby on rails - difference between using JUST 'belongs_to' versus using BOTH 'has_many' and 'belongs_to'?

What is the difference between using just belongs_to on one model versus having has_many on one and belongs_to on another?

As an example:

class Author < ActiveRecord::Base
end

class Book < ActiveRecord::Base
  belongs_to :author
end

versus

class Author < ActiveRecord::Base
  has_many :books
end

class Book < ActiveRecord::Base
  belongs_to :author
end

Thank you.

Solution

guessing each of the methods would facilitate adding a different set of additional methods to the associated class

for ex, if had to guess, with belongs_to, you would, in part, get ability to call association on an instance of Book:

@book.author

with has_many, if I had to guess, you would, in part, be able to call association on instance of Author:

@author.books

and

in case those may be of interest