Firestore Pagination: how to define **unique** 'startAt'-cursor for REST? - firebase

Question

This is a follow up question to an already solved one.
For this previous question an answer was given, how to define a cursor for query-pagination with 'startAt' for REST, that relates to a range of documents. In the example below, the cursor relates to all documents with an 'instructionNumber.stringValue' equal to "instr. 101". According to my testing, this results in skipping of documents.
New question:
How has the cursor to be defined, to not only relate to the stringValue of a field, that the query is ordered by? But instead to a distinct document (usually defined by its document-id)?
"structuredQuery": {
"from": [{"collectionId": "instructions"}],
"where": {
"fieldFilter": {
"field": {
"fieldPath": "belongsToDepartementID"
},
"op": "EQUAL",
"value": {
"stringValue": "toplevel-document-id"
}
}
},
"orderBy": [
{
"field": {
"fieldPath": "instructionNumber"
},
"direction": "ASCENDING"
}
],
"startAt": {
"values": [{
"stringValue": "instr. 101"
}]
},
"limit": 5
}
}
For better understanding, here is the condensed schema of the documents.
{
"document": {
"name": "projects/PROJECT_NAME/databases/(default)/documents/organizations/testManyInstructions/instructions/i104",
"fields":
"belongsToDepartementID": {
"stringValue": "toplevel-document-id"
},
"instructionNumber": {
"stringValue": "instr. 104"
},
"instructionTitle": {
"stringValue": "dummy Title104"
},
"instructionCurrentRevision": {
"stringValue": "A"
}
},
"createTime": "2022-02-18T13:55:47.300271Z",
"updateTime": "2022-02-18T13:55:47.300271Z"
}
}

Answer 1

For a query with no ordering:
"orderBy": [{
"direction": "ASCENDING",
"field": {"fieldPath": "__name__"}
}],
"startAt": {
"before": false,
"values": [{"referenceValue": "last/doc/ref"}]
}
For a query with ordering:
"orderBy": [
{
"direction": "DESCENDING",
"field": {"fieldPath": "instructionNumber"}
},
{
"direction": "DESCENDING",
"field": {"fieldPath": "__name__"}
}
],
"startAt":
{
"before": false,
"values": [
{"stringValue": "instr. 101"},
{"referenceValue": "last/doc/ref"}
]
}
Be sure to use the same direction for __name__ as the previous "orderBy" or it will need a composite index.

Answer 2

To ensure you have identify unique document for starting at you'll always want to include the document ID in your call to startAt.
I'm not sure of the exact syntax for the REST API, but the Firebase SDKs automatically pass this document ID when you call startAt with a DocumentSnapshot.