Mongoose pre.save() async middleware not working as expected
You're using a parallel middleware callback function (with both next and done), but you're not setting the parallel flag in the schema.pre parameters so it's using the serial rules.
So either include the parallel flag in your call:
schema.pre("save", true, function(next, done) { ...Or switch to a serial middleware callback style if that's all you need anyway:
schema.pre("save", function(next) { var self = this; model.findOne({email : this.email}, 'email', function(err, results) { if(err) { next(err); } else if(results) { console.warn('results', results); self.invalidate("email", "email must be unique"); next(new Error("email must be unique")); } else { next(); } });});