How to use the pouchdb-merge.latest function in pouchdb-merge

stores.docStore.get(id, function (err, metadata) {

      if (err || !metadata) {
        return callback(createError(MISSING_DOC, 'missing'));
      }

      var rev;
      if (!opts.rev) {
        rev = getWinningRev(metadata);
        var deleted = getIsDeleted(metadata, rev);
        if (deleted) {
          return callback(createError(MISSING_DOC, "deleted"));
        }
      } else {
        rev = opts.latest ? getLatest(opts.rev, metadata) : opts.rev;
      }

      var seq = metadata.rev_map[rev];

      stores.bySeqStore.get(formatSeq(seq), function (err, doc) {
        if (!doc) {
          return callback(createError(MISSING_DOC));
        }
        /* istanbul ignore if */
        if ('_id' in doc && doc._id !== metadata.id) {
          // this failing implies something very wrong
          return callback(new Error('wrong doc returned'));
        }
        doc._id = metadata.id;
        if ('_rev' in doc) {
          /* istanbul ignore if */

// doc (possibly with _deleted=true) or missing error
      if (!metadata) {
        err = createError(MISSING_DOC, 'missing');
        return finish();
      }

      var rev;
      if (!opts.rev) {
        rev = metadata.winningRev;
        var deleted = isDeleted(metadata);
        if (deleted) {
          err = createError(MISSING_DOC, "deleted");
          return finish();
        }
      } else {
        rev = opts.latest ? getLatest(opts.rev, metadata) : opts.rev;
      }

      var objectStore = txn.objectStore(BY_SEQ_STORE);
      var key = metadata.id + '::' + rev;

      objectStore.index('_doc_id_rev').get(key).onsuccess = function (e) {
        doc = e.target.result;
        if (doc) {
          doc = decodeDoc(doc);
        }
        if (!doc) {
          err = createError(MISSING_DOC, 'missing');
          return finish();
        }
        finish();
      };

txn.txn.objectStore(DOC_STORE).get(id).onsuccess = function (e) {
    var doc = e.target.result;
    var rev;
    if (!opts.rev) {
      rev = (doc && doc.rev);
    } else {
      rev = opts.latest ? getLatest(opts.rev, doc) : opts.rev;
    }

    if (!doc || (doc.deleted && !opts.rev) || !(rev in doc.revs)) {
      callback(createError(MISSING_DOC, 'missing'));
      return;
    }

    var result = doc.revs[rev].data;
    result._id = doc.id;
    result._rev = rev;

    // WARNING: expecting possible old format
    // TODO: why are we passing the transaction in the context?
    //       It's not clear we ever thread these txns usefully
    callback(null, {
      doc: result,

tx.executeSql(sql, sqlArgs, function (a, results) {
      if (!results.rows.length) {
        var err = createError(MISSING_DOC, 'missing');
        return finish(err);
      }
      var item = results.rows.item(0);
      var metadata = safeJsonParse(item.metadata);
      callback(getLatest(rev, metadata));
    });
  }