Expected not to find a document already present for an add error

I am using Meteor 1.8 and the error above is coming because I am using persistent-minimongo.

My use case for persistent minimongo is to use it not only for offline support but to also use it as cache for slow networks. Now the problem happens when user is online and loads the page, persistent-minimongo starts writing data from indexed-db(or similar browser storage) to minimongo while subscriptions too start writing to minimongo, in which case there is a conflict if persitent-minimono wrote any document first in minimongo and we receive the above error.

To fix this I have cloned mongo inside packages folder of my project and made changes to collection.js. Below is the part of the code which was throwing the above error.

    Object.assign(Mongo.Collection.prototype, {
      _maybeSetUpReplication(name, {
        _suppressSameNameError = false
      }) {
        ....
        ....
    
          update(msg) {
            ....
            ....
            
            } else if (msg.msg === 'added') {
              if (doc) {
                throw new Error("Expected not to find a document already present for an add");
              }
              self._collection.insert({ _id: mongoId, ...msg.fields });*
            }

...
...

I changed the above part of the code to not thorw an error as shown below.

else if (msg.msg === 'added') {
          if (doc) {
            self._collection.remove(mongoId)
          }
          self._collection.insert({ _id: mongoId, ...msg.fields });

Note: I do not want to make change to meteor core packages but I could not find any better solution.

The above change fixed my issue but I am a little worried about the repercussion of the above change. Want to understand as to why an error is thrown rather than replacing the document.

Any help would be appreciated. Thanks in advance!!

Because executing an automated mutation based only on the document _id is potentially dangerous: there’s nothing to indicate that it’s the “same” document. It’s much better not to do anything destructive.

However, if you’re confident that you will never see this situation in your app, or that if you do see it, it will be safe to remove and insert, then go for it.

IIRC, people have reported getting that error when inserting large numbers of documents quickly.