Add per-instance referrers endpoint (e.g. /data/instance/{id}/referrers)

[adamjohnwright]

Background

WebsiteAngular's beta /content/schema/instance/:dbId view is missing the "Referrals" section that the legacy Joomla page at https://reactome.org/content/schema/instance/browser/{dbId} renders. That section lists every other instance that references the current one, grouped by attribute name. Example today on prod:

Referrals
(hasModifiedResidue) — [EntityWithAccessionedSequence:R-XTR-159718] f10 [Golgi lumen], [EntityWithAccessionedSequence:R-XTR-159774] …

ContentService currently only exposes class-level referrers via GET /data/schema/{className}/referrals — which returns metadata about which attributes on which classes can reference instances of the named class. That's enough to render the Properties tab's "Referrals" section on /content/schema/{className}, but not enough to render per-instance referrers without an N+1 query storm from the frontend.

Ask

Add a new endpoint along these lines:

GET /data/instance/{id}/referrers

Response shape (suggested):

[
  {
    "attribute": "hasModifiedResidue",
    "fromClass": "EntityWithAccessionedSequence",
    "referrers": [
      { "dbId": 159718, "stId": "R-XTR-159718", "displayName": "f10 [Golgi lumen]", "schemaClass": "EntityWithAccessionedSequence" },
      ...
    ]
  },
  ...
]

Backed by a Cypher query roughly like:

MATCH (n)-[r]->(target)
WHERE target.dbId = $id
RETURN type(r) AS attribute, labels(n) AS classes, n.dbId, n.stId, n.displayName

…plus the usual apoc.coll.disambiguate / grouping to bucket by attribute.

Why

Without this, the WebsiteAngular beta can't replace prod's instance browser page — there is no frontend-only workaround that's fast enough (would require N+1 lookups across every potential referrer class for every page render).

Consumer

• Repo: https://github.com/reactome/WebsiteAngular
• File: projects/website-angular/src/app/content/schema/instance-browser/instance-browser.component.ts
• Will fetch this endpoint after getInstance resolves and render the result below the existing attribute table.