eliminate need for has_many :through redundancy

I'm surprised this is not yet reported/requested, though perhaps it is and I just couldn't find it.

I think that has_many :through should create the :through association if it's not already declared.

Example:

class User < ActiveRecord::Base
  has_many :memberships
  has_many :groups, :through => :memberships
end

Why do I need to declare :memberships twice? Shouldn't this suffice?

class User < ActiveRecord::Base
  has_many :groups, :through => :memberships
end

Even if the memberships association is more complex, this could be wrapped in a hash

class User < ActiveRecord::Base
  has_many :groups, :through => {
    :memberships, :dependent => :destroy
  }
end

In terms of expressiveness, I can see the value in declaring the join model explicitly if that join model has significance, but sometimes join models are only really useful on one side of the join, while on the other side it's just noise.

Thoughts?