You might want to use 'destroy' instead of 'delete', as 'delete' skips all ActiveRecord callbacks (i. e., if you had a dependent: :destroy callback for post comments, using 'delete' would leave them intact)
I'd suggest deleting or dealing with those records anyway, regardless of whether you added FKs! But... if you don't add FKs, you've got bigger issues to deal with :(
this is why i prefer using "on delete cascade" in schemas, no need to transmit lots of data over the wire and instantiate thousands of slow activerecord objects when deleting stuff.