Automatic join model not created

gistfile1.r

== Activity Model ==
has_many :rule_assignments, :dependent => :destroy   # RuleAssignment has activity_id, and rule_type, rule_id
has_many :be_located_in_rules, :through => :rule_assignments, :source => :rule, :source_type => 'BeLocatedInRule'

== BeLocatedInRules Controller ==
def create
  @rule = @activity.be_located_in_rules.build(params[:be_located_in_rule])
  ...

  @rule.save # successfully create the BeLocatedInRule, but no join model (RuleAssignment)