Merging of discrimintor schema "_indexes" duplicates when both 'from' and 'to' are schema instances

[shlomiassaf]

Prerequisites

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

Mongoose version

8.3.4

Node.js version

18.2

MongoDB server version

6

Typescript version (if applicable)

5.4.5

Description

When merging a discriminator schema, when both source and target are Schema instances the following logic is applied:

1. The source schema (from) is cloned
2. The cloned source from step 1 is merged into the target (to)

mongoose/lib/helpers/discriminator/mergeDiscriminatorSchema.js

Line 64 in 3526799

schemaMerge(to[key], from[key].clone(), true);

Step 1 (clone) will also close the _indexes property of the Schema creating new copied for the array and the index definitions.

mongoose/lib/helpers/clone.js

Lines 175 to 184 in 3526799

	function cloneArray(arr, options) {
	let i = 0;
	const len = arr.length;
	const ret = new Array(len);
	for (i = 0; i < len; ++i) {
	ret[i] = clone(arr[i], options, true);
	}
	return ret;
	}

Next, the merge operation will concat the _indexes from source and target effectivly duplicating the _indexes on every clone.

mongoose/lib/helpers/schema/merge.js

Line 34 in 3526799

s1._indexes = s1._indexes.concat(s2._indexes || []);

For multiple discriminators and nested schema's this can lead to memory leak/overflow and other mongo implications.

Expected Behavior

when merging discriminator schemas, the clone phase should be able to optionally ignore _indexes cloning and the "merge" itself should clone and concat properly

[shlomiassaf]

Not an issue