Using a has_list and has_one will never work together, one is for outgoing relationships and the other for one relationship with linked list.
However, it would be really cool if th following example works.
class A
include Neo4j::NodeMixin
has_list :a2b
end

class B
has_list(:b2a).from(A)
end

I guess it does not work - have never tried.

It should be
class B
has_list(:b2a).from(A, :a2b)
end

You are right, of course. I got the impression that has_list was just has_n with an order parameter, but Neoclipse tells me otherwise. Thanks, and sorry for raising an issue about it.

In an almost related comment; assigning a node to a has_one relationship does not delete the previous relationship. This does not seem to me like the behaviour most people would expect, but again, I'm new to the library. As an example, the following script prints "1 2", whereas simply "2" might be more intuitive:

require 'rubygems'
require 'neo4j'

class B
  include Neo4j::NodeMixin
  property :name
end

class A
  include Neo4j::NodeMixin
  has_one(:a2b).to(B)
end

Neo4j::Transaction.run do
  a = A.new
  b1 = B.new(:name => '1')
  b2 = B.new(:name => '2')
  a.a2b = b1
  a.a2b = b2

  puts a.a2b.name
  a.a2b_rel.del
  puts a.a2b.name
end

Yes, you are right. I'm not sure what the expected behaviour would be.
Should it be a more functional way - you could only assign it once or a get an exception, or should we allow to assign it several times. If the assign it several times should we then delete the old relationship - hmm

Funny last sentance ... - If we reassign the relationship what should happen with the old relationship - should it get deleted ?

I think it should be deleted without throwing an exception. If the current state is node polygamy, throwing an exception would be like forbidding divorce. Seems more natural to me just to let the node replace one relationship with the other. My code is currently using this for has_one assignments:

while rel = a.a2b_rel
  rel.del
end
a.a2b_rel = new_partner_node

Ok, see http://neo4j.lighthouseapp.com/projects/15548-neo4j/tickets/106-has_one-should-replace-old-relationship

Cool. Thanks!