gistfile1.rb

# Assuming AR test models are present...
# The salient bits are:
#
# class Developer < ActiveRecord::Base
#   has_many :audit_logs
# end
#
# class AuditLog < ActiveRecord::Base
#   belongs_to :developer, :validate => true
# end

d = Developer.new :name => 'Murray'
d.audit_logs.build :message => 'What AR does'
d.audit_logs.first.developer #=> nil
d.valid? #=> true

d2 = Developer.new :name => 'Hans'
d2.audit_logs.build :message => 'Simulating what :inverse_of does', :developer => d2
d2.audit_logs.first.developer #=> d2
d2.valid? #=> BOOM! Stack Trace Too Deep

# It's even worse in reality as the Developer model has a :before_create that 
# will populate audit_logs with the first AuditLog instance. You can't create or
# save any Developer instances!