title: Validation in a Domain Driven Design
body: <p>How do you deal with validation on complex aggregates in a domain driven design? Are you consolidating your business rules/validation logic?</p>
  <p>I understand argument validation and I understand property validation which can be attached to the models themselves and do things like check that an email address or zipcode is valid or that a first name has a minimum and maximum length.</p>
  <p>But what about complex validation that involves multiple models? Where do you typically place these rules &amp; methods within your architecture? And what patterns if any do you use to implement them?</p>