Chapter 37 showed the basic case – this chapter covers ERROR cases when linking and shows the path via the nested endpoint from chapter 39.

Linking with an invalid IRI

curl -k -X POST https://localhost/api/tasks \
  -H 'Content-Type: application/json' \
  -d '{"title": "Test", "project": "/api/projects/99999"}'

{
  "violations": [
    {"propertyPath": "project", "message": "Item not found for \"/api/projects/99999\"."}
  ]
}

Status 422 – API Platform RESOLVES the IRI BEFORE saving, and reports a VALIDATION error if the target does NOT exist, in the EXACT same violations format from chapter 20.

Linking with the wrong resource type

curl -k -X POST https://localhost/api/tasks \
  -H 'Content-Type: application/json' \
  -d '{"title": "Test", "project": "/api/tags/1"}'

ALSO 422 – /api/tags/1 is a SYNTACTICALLY valid IRI, but of the WRONG type (Tag instead of Project). API Platform CHECKS the target type, not just its existence.

Creating via the nested endpoint

The sub-resource path from chapter 39 (/api/projects/{projectId}/tasks) only had GetCollection in our example – for POST via this path, the operations list would need to be EXTENDED with new Post():

operations: [new GetCollection(), new Post()],

curl -k -X POST https://localhost/api/projects/1/tasks \
  -H 'Content-Type: application/json' \
  -d '{"title": "Prepare deployment"}'

WITHOUT an explicit project field in the body – API Platform sets it AUTOMATICALLY from the URL (projectId), since Link (chapter 39) ALREADY knows this connection.