Simplified test case with associations fixed

test.rb

require 'dm-core'

DataMapper.setup(:default, 'sqlite::memory:')

class Project
  include DataMapper::Resource
  property :id, Serial

  has n, :members
  has n, :users, :through => :members
end

class Member
  include DataMapper::Resource
  
  belongs_to :user, :key => true
  belongs_to :project, :key => true
end

class User
  include DataMapper::Resource
  property :id, Serial

  has n, :members
  has n, :projects, :through => :members
end

DataMapper.finalize
require  'dm-migrations'
DataMapper.auto_migrate!

user1 = User.new
proj = Project.new :users => [user1]
user2 = User.new
proj.users << user2 # you can also still use the << operator
puts proj.save.inspect
puts proj.users.inspect