gistfile1.builder

#given the following models---------------------------

class Parent
  has_many :children
end

class Child
  belongs_to :parent       #column: parent_id in the db
end


#Set up some relationships-----------------------------

#Jimmy has a dad:
dad = Parent.new           #let's say he gets id=1
jimmy = Child.new
jimmy.parent_id = dad.id   #stores '1' parent_id column
jimmy.save!

#Jonny is an orphan :(
jonny = Child.new


#If want to access the parent id of a child--------------
#We can either do child.parent.id, or child.parent_id

#This works find for Jimmy
jimmy.parent.id            #=>1   (yup)
jimmy.parent_id            #=>1   (yup)

#but if we try this on Jonny (who has no parent)
jonny.parent_id            #=>nil  (as expected)
jonny.parent.id            #=>4    <<(wtf?!)