Mongoose: How to validate length of array based on another documents variable - node.js

Question

I am trying to do validation within the schema to validate the length of an array based on another documents length of its array. Suppose:
const Level = mongoose.Schema({
level: {
type: Array,
required: true,
items: {
exercise: {
type: mongoose.Schema.Types.ObjectId,
ref: "Exercise",
required: true,
},
accuracyThreshold: { type: Number, required: true },
timeThreshold: { type: Number, required: true },
},
},
});
const UserLevelProgress = mongoose.Schema({
user: {
type: mongoose.Schema.Types.ObjectId,
ref: "User",
required: true,
},
level: {
type: mongoose.Schema.Types.ObjectId,
ref: "Level",
required: true,
},
progress: {
type: Array,
required: true,
validate: {
validator: (progress) => {
return progress.length === level.level.length; // level.level.length is not valid
},
message: () => "level progress legnth is invalid!",
},
items: {
accuracyScore: { type: Number, required: true },
timeScore: { type: Number, required: true },
},
},
});
module.exports = mongoose.model("UserLevelProgress", UserLevelProgress);
module.exports = mongoose.model("Level", Level);
I have a validator function in UserLevelProgress, but obviously level is not valid object. How do I get access to this object?

Answer 1

The short answer is: You could implement this validation logic, but is it really a good idea to do this?
Downsights
validation is taking place in the pre('save') hook. This means that once you initially save of update a document, the validation will run.
since Level documents are stored in a different collection, the values (including length of the array) can change without UserLevelProgress documents noticing. This way your validator would not rerun (as said before it's run in pre('save') hook and your documents would technically be not valid anymore.
Better approach
Check if Level schema can be included as a nested document, instead of a referenced one. This way validation would be much easier and data will not be able to become invalid unnoticed.
Implementation if you still want to follow your intended approach
validate: {
validator: async function(progress) {
const level = await Level.findById(this.level);
return progress.length === level.level.length; // level.level.length is not valid
},
message: () => "level progress legnth is invalid!",
},