Schema.add method doesn't copy indexes

[JorgeSilva7]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.6.5

Node.js version

14.18.0

MongoDB server version

4.2.21

Typescript version (if applicable)

4.8.4

Description

When i use Schema.prototype.add() method for copying schema to an other schema, the indexes aren't copied.

Steps to Reproduce

I have a test schema

const test = new Schema({
   name: String
}):

...someMethods, statics...

test.index({name: 1});

And another schema called 'extended' and i want to add test schema in extended with add() method like the documentation says "You can also add() another schema and copy over all paths, virtuals, getters, setters, indexes, methods, and statics."

const extended = new Schema({
   property: String
});

extended.add(test);

But extended schema doesn't have the index name

Expected Behavior

When i call extended.indexes() the returned array has the index {name: 1}

[hasezoey]

And other schema called 'extended' and i want to add test schema in extended with add() method like the documentation say "You can also add() another schema and copy over all paths, virtuals, getters, setters, indexes, methods, and statics."

i think the documentation needs to be updated, because in the description it says Adds key path / schema type pairs to this schema. while in the code comments it says You can also 'add()' another schema and copy over all paths, virtuals, getters, setters, indexes, methods, and statics. [Schema.prototype.add()](https://mongoosejs.com/docs/api/schema.html#schema_Schema-add)

---

as a workaround, if test is a base-schema and you dont need to add other schemas, consider using .clone

[JorgeSilva7]

But currently the add() method copy paths, methods, statics, virtuals, getters, setters. Only the indexes don't be copied.

[IslandRhythms]

I did some digging. If you define the index in the schema and then call add, if will be added correctly. If you use the index method it will not be added correctly.

const mongoose = require('mongoose');

const testSchema = new mongoose.Schema({
  name: {type: String}
});

const otherSchema = new mongoose.Schema({
  age: { type: Number}
});

otherSchema.index({ age: 1 })

testSchema.add(otherSchema);

console.log(testSchema);
console.log(testSchema.tree)
console.log(testSchema.tree.age)

Marking as a bug because if the docs say it suppose to do something, it should be doing it. Especially if the code is written so.

[JorgeSilva7]

Finally i used .clone() method and .add() with .set() methods for add paths and options.

testSchema.index({...someIndex});

const otherSchema = testSchema.clone();
otherSchema.add({...paths})
otherSchema.set(someOptions)

Now otherSchema has the indexes of testSchema