HABTM using two columns

Hi all,

I was wondering whether anyone could guide me to docu and code
examples how to create and use a has_and_belongs_to_many relationship
with a joining table where TWO columns are used to identify the
relation to TWO alternative tables.

Example:
table 1: mothers [mother_id, name]
table 2: fathers [father_id, name]
table 3: children [child_id, name]
joining table 4: children_parent [child_id, parent_id, parent_type]

'parent type' can be either 'mother' or 'father', and depending on
this, the 'parent_id' links to an entry in either the father or mother
table.
How do you define this in Rails and how do you query it when showing
the child data?

  Thanks for any suggestions,
     Flexer.

Before you get too far, I'd reconsider the models you're using...

Mothers, Fathers, and Children are all people, and ideally a Person is
in your database just once (the person who is a child can also be a
father, etc).

What makes a Person a Mother, or Father (or grandmother, grandfather,
etc) is just a relationship between that Person and another.

Factor the relationships out into something like:

Relation
  person_1
  relation_type
  person_2
where "person_1" is the "relation_type" of "person_2"

and you can start supporting Mother, Father, Sister, Brother, Friend,
Enemy, etc, etc.

Sounds like you want Polymorphic Associations: see
http://wiki.rubyonrails.org/rails/pages/HowToUsePolymorphicAssociations