Does Mongoose provide access to previous value of property in pre('save')? - mongodb

Question

I'd like to compare the new/incoming value of a property with the previous value of that property (what is currently saved in the db) within a pre('save') middleware.
Does Mongoose provide a facility for doing this?

Answer 1

The accepted answer works very nicely. An alternative syntax can also be used, with the setter inline with the Schema definition:
var Person = new mongoose.Schema({
name: {
type: String,
set: function(name) {
this._previousName = this.name;
return name;
}
});
Person.pre('save', function (next) {
var previousName = this._previousName;
if(someCondition) {
...
}
next();
});

Answer 2

Mongoose allows you to configure custom setters in which you do the comparison. pre('save') by itself won't give you what you need, but together:
schema.path('name').set(function (newVal) {
var originalVal = this.name;
if (someThing) {
this._customState = true;
}
});
schema.pre('save', function (next) {
if (this._customState) {
...
}
next();
})

Answer 3

I was looking for a solution to detect changes in any one of multiple fields. Since it looks like you can't create a setter for the full schema, I used a virtual property. I'm only updating records in a few places so this is a fairly efficient solution for that kind of situation:
Person.virtual('previousDoc').get(function() {
return this._previousDoc;
}).set(function(value) {
this._previousDoc = value;
});
Let's say your Person moves and you need to update his address:
const person = await Person.findOne({firstName: "John", lastName: "Doe"});
person.previousDoc = person.toObject(); // create a deep copy of the previous doc
person.address = "123 Stack Road";
person.city = "Overflow";
person.state = "CA";
person.save();
Then in your pre hooks, you would just need to reference properties of _previousDoc such as:
// fallback to empty object in case you don't always want to check the previous state
const previous = this._previousDoc || {};
if (this.address !== previous.address) {
// do something
}
// you could also assign custom properties to _previousDoc that are not in your schema to allow further customization
if (previous.userAddressChange) {
} else if (previous.adminAddressChange) {
}

Answer 4

Honestly, I tried the solutions posted here, but I had to create a function that would store the old values in an array, save the values, and then see the difference.
// Stores all of the old values of the instance into oldValues
const oldValues = {};
for (let key of Object.keys(input)) {
if (self[key] != undefined) {
oldValues[key] = self[key].toString();
}
// Saves the input values to the instance
self[key] = input[key];
}
yield self.save();
for (let key of Object.keys(newValues)) {
if (oldValues[key] != newValues[key]) {
// Do what you need to do
}
}

Answer 5

What I do is use this.constructor within the pre-save route to access the current value in the database.
const oldData = this.constructor.findById(this.id)
You can then grab the specific key you're looking for from the oldData to work with as you see fit :)
let name = oldData.name
Note that this works well for simple data such as strings, but I have found that it does not work well for subschema, as mongoose has built in functionality that runs first. Thus, sometimes your oldData will match your newData for a subschema. This can be resolved by giving it it's own pre-save route!