How can I get the previous data of a document before it is updated via replicated data

Mobile Couchbase Lite
1 
Retained<C4Document> putDocument(const C4DocPutRequest& rq, size_t* outCommonAncestorIndex,
                                         C4Error* outError) override {
            dbImpl()->mustBeInTransaction();
            if ( rq.docID.buf && !C4Document::isValidDocID(rq.docID) ) error::_throw(error::BadDocID);
            if ( rq.existingRevision || rq.historyCount > 0 ) AssertParam(rq.docID.buf, "Missing docID");
            if ( rq.existingRevision ) {
                AssertParam(rq.historyCount > 0, "No history");
            } else {
                AssertParam(rq.historyCount <= 1, "Too much history");
                AssertParam(rq.historyCount > 0 || !(rq.revFlags & kRevDeleted),
                            "Can't create a new already-deleted document");
                AssertParam(rq.remoteDBID == 0, "remoteDBID cannot be used when existingRevision=false");
            }

            int                  commonAncestorIndex = 0;
            Retained<C4Document> doc;
            if ( rq.save && isNewDocPutRequest(rq) ) {
                // As an optimization, write the doc assuming there is no prior record in the db:
                std::tie(doc, commonAncestorIndex) = putNewDoc(rq);
                // If there's already a record, doc will be null, so we'll continue down regular path.
            }
            if ( !doc ) {
                if ( rq.existingRevision ) {
                    // Insert existing revision:
                    doc = getDocument(rq.docID, false, kDocGetAll);
                    
                    auto previousData = doc->bodyAsJSON();
                    C4Error err;
                    
                    commonAncestorIndex = doc->putExistingRevision(rq, &err);
                    if ( commonAncestorIndex < 0 ) {
                        throwIfUnexpected(err, outError);
                        doc                 = nullptr;
                        commonAncestorIndex = 0;
                    }
                } else {
                    // Create new revision:
                    alloc_slice docID = (rq.docID.buf) ? alloc_slice(rq.docID) : C4Document::createDocID();

                    slice parentRevID;
                    if ( rq.historyCount > 0 ) parentRevID = rq.history[0];

                    doc = getDocument(docID, false, kDocGetAll);
                    auto previousData = doc->bodyAsJSON();
                    C4Error err;
                    if ( !doc->checkNewRev(parentRevID, rq.revFlags, rq.allowConflict, &err)
                         || !doc->putNewRevision(rq, &err) ) {
                        throwIfUnexpected(err, outError);
                        doc = nullptr;
                    }
                    commonAncestorIndex = 0;
                }
            }

            Assert(commonAncestorIndex >= 0, "Unexpected conflict in c4doc_put");
            if ( outCommonAncestorIndex ) *outCommonAncestorIndex = commonAncestorIndex;
            return doc;
        }

here’s the method which is calling when replication pull insert any revision.
I am calling this auto previousData = doc->bodyAsJSON(); to get the body before updating it’s content but it’s returning null.

2

Screenshot 2024-02-16 at 10.05.09 AM
Screenshot 2024-02-16 at 10.05.09 AM2186×1064 160 KB

3

could you please explain it.

4

Looks like your code is doing a local write/put. you could do this. 2 writes.

Screenshot 2024-02-16 at 10.44.06 AM
Screenshot 2024-02-16 at 10.44.06 AM2198×1002 155 KB

5

are you talking about creating separate db?

6

Yea , or put it into a separate scope/collection in the same DB1. I would putt a TTL on the old revs 7 days ->30Days , how every long they are useful. If you keep them for a long time 3 month to 18 months. I would put them in a separate db2.

7

Can’t we simply send the previous doc before updating it locally. If we need to maintain separate db in that case we don’t need to work on core library. Also I think it will make the database size 2x larger.

8

if it where me I would not make it so I have to do a READ before I WRITE. It’s just simpler to write twice. Will it make your second db X 2 bigger… Yes , but you have a complete audit trail of the document. for xyz days/weeks/months.

9

Actually my concern is if I use multiple database then syncing those data will take for time. And I don’t want to do that that’s why I was asking for reading the value before writing while new replicated data comes from server.