Activerecord model belongs_to and validates_associated duplicate validation

In one of my models I have these lines :

belongs_to :user, :validate => true

validates_associated :user

When I remove the 2nd one, my tests run noticeably faster. Is the
above causing validations to be run twice ?

And if it is a duplication, are there any differences, and which
method is best ?

Also, is there some reason why validates_associated does not throw an
exception on nil ?

That’s just the way it’s designed to work. If you want to make sure it’s
null, you’ll need to also use validates_presence_of or validates
=> true