We di-sected the old accepts_nested_attributes_for_apis_for - it does two things: 
 * mapping association-names on the corresponding _id (e.g. planning_element.priority get's mapped onto planning_element.priority_id
 * mark the associations with attr_accessible 

The first one is as far as I know still needed in the api, as the attributes come in with priority= instead of priority_id=. The second one does not work well together with strong_parameters/permit-logic: that's why this has been split here, the acts_as...- been renamed, and the permission-logic moved into the planning-element-controller: Let's revisit this here, when we deal with [#1919](https://www.openproject.org/issues/1919).