When using JPA identity store, there is no way to query groups by their parent group.
Example:

• We have one group called RootGRP with no parent, and groups SubGrpA and SubGrpB whoose parent is group RootGRP.

• When trying to get groups that has group RootGRP as parent the following code

  {
  	Group rootGroup = identityManager.getGroup("/RootGRP");
  	List<Group> groups = identityManager.createIdentityQuery(Group.class).setParameter(Group.PARENT, rootGroup).getResultList();
  }
  

  fails with exception:
  org.picketlink.idm.IdentityManagementException: PLIDM000055: Could not query IdentityType using query [org.picketlink.idm.query.internal.DefaultIdentityQuery@330112b2].

• When trying to put parent group id as parameter:

  {
  	Group rootGroup = identityManager.getGroup("/RootGRP");
  	List<Group> groups = identityManager.createIdentityQuery(Group.class).setParameter(Group.PARENT, rootGroup.getId()).getResultList();
  }
  

  Nothing is returned.