Question

class Foo < ActiveRecord::Base {
  has_one :bar
}

Does bar table in schema need a foreign key reference to foo_id?

Answer 1

http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

Yes. bar would have a FK to Foo

class Bar < ActiveRecord::Base {
  belongs_to :foo 
 }

schema would be:

CREATE TABLE bars (
  id int,
  foo_id int,
  .....

}

has_one and has_many both define FK relationships, has_one returns a singular object, has_many returns a collection.