I would like to know what i ve done wrong in my association.
I have 4 Models.

Actually i want to get Particular user's company,and which department he
belongs and what role he has...
These inforamtions i would like to get.
So i made association.

class CompanyInfo < ActiveRecord::Base

The setup seems to be right, assuming
there is a company_info_id in the user_info table.

Are the records in the database ok and have the
right id?
What code do you use to get the CompanyInfo?
Something like:

@user_info = UserInfo(:first)

That should allow to acces it like that:

It seems all of your "has_one" should be has_many. A company, role and
department should have many users. That might reduce problems with
your associations.