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

Question

I am trying to use a cursor and 'startAt', to paginate REST requests to Firestore. According to the Paginate-documentation, the cursor should equal to the last document of the previous query. As the REST-documentation is without an example, I tried to run it by inserting an entire document as cursor in the startAt-object; like this:
POST https://firestore.googleapis.com/v1/PROJECT-NAME/databases/(default)/documents/organizations/testManyInstructions:runQuery
{
"structuredQuery": {
"from": [
{
"collectionId": "instructions"
}
],
"where": {
"fieldFilter": {
"field": {
"fieldPath": "belongsToDepartementID"
},
"op": "EQUAL",
"value": {
"stringValue": "toplevel-document-id"
}
}
},
"orderBy": [
{
"field": {
"fieldPath": "instructionNumber"
},
"direction": "ASCENDING"
}
],
"startAt": {
"values": [{
"document": {
"name": "projects/PROJECT-NAME/databases/(default)/documents/organizations/testManyInstructions/instructions/i0",
"fields": {
"checkbox": {
"booleanValue": false
},
"retrainTimespanDays": {
"integerValue": "365000"
},
"approvedByName": {
"stringValue": ""
},
"instructionNumber": {
"stringValue": "instr. 0"
},
"instructionCurrentRevision": {
"stringValue": "A"
},
"instructionCurrentRevisionPublishingDate": {
"timestampValue": "1999-01-01T00:00:00Z"
},
"instructionFileURL": {
"stringValue": ""
},
"instructionTitle": {
"stringValue": "dummy Title0"
},
"instructionFileUploadDate": {
"timestampValue": "1999-01-01T00:00:00Z"
},
"belongsToDepartementID": {
"stringValue": "toplevel-document-id"
},
"approvedByEmailAdress": {
"stringValue": ""
}
},
"createTime": "2022-02-18T13:55:42.807103Z",
"updateTime": "2022-02-18T13:55:42.807103Z"
}
}
]
},
"limit": 5
}
}
without the "startAt"-Object, the following code works fine and returns 5 documents.
with the "startAt"-Object, this error is returned:
{
"error": {
"code": 400,
"message": "Invalid JSON payload received. Unknown name \"document\" at 'structured_query.start_at.values[0]': Cannot find field.",
"status": "INVALID_ARGUMENT",
"details": [
{
"#type": "type.googleapis.com/google.rpc.BadRequest",
"fieldViolations": [
{
"field": "structured_query.start_at.values[0]",
"description": "Invalid JSON payload received. Unknown name \"document\" at 'structured_query.start_at.values[0]': Cannot find field."
}
]
}
]
}
}
]
Please advise, how to set the cursor in the startAt-object correctly.

Answer 1

I've run a similar query using offset instead of startAt, so I tried modifying and got it to work. This is the rest api documentation I used.
startAt requires a Cursor object which is an array of Values.
https://firebase.google.com/docs/firestore/reference/rest/v1/StructuredQuery
https://firebase.google.com/docs/firestore/reference/rest/v1/Cursor
https://firebase.google.com/docs/firestore/reference/rest/Shared.Types/ArrayValue#Value
I would have preferred an example as well!
"startAt": {
"values": [{
"stringValue": "Cr"
}]
},
"orderBy": [{
"field": {
"fieldPath": "Summary"
}
}],
Good luck!