Question

In the following instance, what would the difference be between using belongs_to :mother and has_one :mother for the Chlid class? I’ve been reading the Rails documentation on this and I can’t see how either one would make a difference apart from the semantics involved with reading it.

From what I can tell, the various associations add extra methods to each class, but I haven’t been able to find the documentation to list per association what the methods are and what they do.

class BiologicalMother < ActiveRecord::Base
  has_many :children
end

class Child < ActiveRecord::Base
  belongs_to :biological_mother
end

Answer 1

In your case the has_many belongs_to is the right approach not just semantically but as how rails works. The foreign key is always stored in the belongs_to part of the association.
A valid has_one scenario could be like having a Purchase model which has_one BillingAddress.

example:

class Purchase
   has_one :billing_address
end

class BillingAddress
   belongs_to :purchase #this holds the foreign key - purchase_id
end

Regarding your case, you cant use has_many in one side and has_one at the other side of the association because the belongs_to part holds the foreign key always.

Let me know if this works for you.