I am assuming that you also have the three classes that correspond to the tables set up with a has_many relationship...
So,
def Table_a < ActiveRecord::Base has_many :table_as, :dependent => :destroy has_many :table_us, dependent => :destroy end
def Table_b < ActiveRecord::Base belongs_to :table_a end
def Table_u < ActiveRecord::Base belongs_to :table_a end
The :dependent => :destroy is all you need in order to be able to do...
Table_a.find(:first).destroy
...and have all Table_u and Table_c entries that reference the first entry in Table_a destroyed.
Have a look at the options here:
http://api.rubyonrails.org/classes/ActiveRecord/Associations/ ClassMethods.html#M000530
-christos