I am trying to use Mongoose pre and post hooks in my MongoDB backend in order to compare the document in its pre and post-saved states, in order to trigger some other events depending on what's changed. So far however I'm having trouble getting the document via the Mongoose pre hook.

According to the docs "pre hooks work for both doc.save() and doc.update(). In both cases this refers to the document itself... ". So I here's what I tried. First in my model/schema I have the following code:

let Schema = mongoose
  .Schema(CustomerSchema, {
    timestamps: true
  })
  .pre("findOneAndUpdate", function(next) {
    trigger.preSave(next);
  })
  // other hooks
}

... And then in my triggers file I have the following code:

exports.preSave = function(next) {
  console.log("this: ", this);
  }
};

But this is what logs to the console:

this: { preSave: [Function], postSave: [AsyncFunction] }

So clearly this didn't work. This didn't log out the document as I was hoping for. Why is this not the document itself here, as the docs themselves appear to indicate? And is there a way I can get a hold of the document with a pre hook? If not, is there another approach people have used to accomplish this?

You can't retrieve the document in the pre hook.

According to the documentation pre is a query middleware and this refers to the query and not the document being updated.