Rails: When using 'uniqueness' constraint for validations

http://ruby.11.x6.nabble.com/validates-uniqueness-of-only-if-present-td2704659.html

nil == nil
returns true in Ruby

So, if we are using 'uniqueness' constraint, it is important to remember to also use
:allow_blank => true
if we are expecting 'nil' values for that column

E.g.
validates_uniqueness_of  :c_id, :allow_blank => true