preliminary commit 0.5 API with relations / untested after entity-relation rename, some changes still in queue

app/models/relation_member.rb

@@ -0,0 +1,30 @@
+class RelationMember < ActiveRecord::Base
+  set_table_name 'current_relation_members'
+
+  # problem with RelationMember is that it may link to any one 
+  # object (a node, a way, another relation), and belongs_to is
+  # not flexible enough for that. So we do this, which is ugly,
+  # but fortunately rails won't actually run the SQL behind that
+  # unless someone really accesses .node, .way, or
+  # .relation - which is what we do below based on member_type.
+  # (and no: the :condition on belongs_to doesn't work here as
+  # it is a condition on the *referenced* object not the 
+  # *referencing* object!)
+  
+  belongs_to :node, :foreign_key => "member_id"
+  belongs_to :way, :foreign_key => "member_id"
+  belongs_to :relation, :foreign_key => "member_id"
+
+  # so we define this "member" function that returns whatever it
+  # is.
+ 
+  def member()
+    return (member_type == "node") ? node : (member_type == "way") ? way : relation
+  end
+
+  # NOTE - relations are SUBJECTS of memberships. The fact that nodes, 
+  # ways, and relations can be the OBJECT of a membership,
+  # i.e. a node/way/relation can be referenced throgh a
+  # RelationMember object, is NOT modelled in rails, i.e. these links
+  # have to be resolved manually, on demand. 
+end