Firestore 分页：如何为 REST 定义 **unique** 'startAt'-cursor？

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 .