What you’ve got is equivalent to this (wrapped for clarity):
validates :email, :presence => true,
:uniqueness => { :allow_blank => true, :case_sensitive => false }
That’s a little silly though since if you’re requiring presence, then that’s going to “invalidate” the :allow_blank clause to :uniqueness.
It makes more sense when you switch to using other validators.. say… format and uniqueness, but you don’t want any checks if it’s blank. In this case, adding a “globally applied” :allow_blank makes more sense and DRY’s up the code a little bit.
This…
validates :email, :format => {:allow_blank => true, ...},
:uniqueness => {:allow_blank => true, ...}
can be written like:
validates :email, :allow_blank => true, :format => {...}, :uniqueness => {...}