Small fix to handle parent ID strategy mappings

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | ..
| Fixed tickets | 
| License       | MIT
| Doc PR        | n/a

Uses ->getDocumentId instead of ->getIdentifierValue etc. to determine the ID when finding out if a node already exists.