title: How to GET associated ActivityDefinitions for a specific PlanDefinition FHIR v1.9.0
body: <p>FHIR stu3</p>
  
  <p>tried this:
  <a href="http://fhirtest.uhn.ca/baseDstu3/PlanDefinition/20630?_revinclude=ActivityDefinition" rel="nofollow noreferrer">http://fhirtest.uhn.ca/baseDstu3/PlanDefinition/20630?_revinclude=ActivityDefinition</a>
  based on the example from : 
  <a href="http://build.fhir.org/search.html#include" rel="nofollow noreferrer">http://build.fhir.org/search.html#include</a>
  but it returned 400 Bad Request</p>
  
  <p>thanks</p>