fix: remove conflicting child projections when parent path excluded

[Himanshu197200]

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Fix: Prevent Parent–Child Projection Collisions in Mongoose (#12798 )

Overview

This pull request fixes an issue where Mongoose generates invalid MongoDB projections when both a parent path and its nested child path are excluded. This occurs when combining:

• Schema-level select: false fields
• User-level .select('-parent') projections

MongoDB does not allow excluding both a parent and its child paths at the same time, resulting in a Path collision error.

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The Problem

Given the following schema:

const BarSchema = new Schema({
  name: String,
  subd: {
    raw: { type: String, select: false },
    clean: String
  }
});

Running this query:

Bar.find().select('-subd');

Currently results in this projection:

{ 
  subd: 0,
  'subd.raw': 0   
}

MongoDB error:

Path collision: Cannot project both 'subd' and 'subd.raw'

---

The Fix

This PR ensures that nested child exclusions are removed automatically when the parent path is already excluded.

Correct behavior after fix:

{
  subd: 0
}

A new helper — removeConflictingProjections() — cleans up the projection before it is returned and forwarded to MongoDB.

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Implementation Details

• Updated parseProjection.js

• Added removeConflictingProjections():

  • Detects excluded parent paths
  • Removes child-level exclusions that would cause collisions

• Ensures compatibility with:

  • Schema-level select: false
  • Query-level .select() usage
  • Existing projection logic

---

Tests Added

A new test validates that:

• Excluding a parent path removes nested exclusions
• Schema-level defaults do not conflict with user projections
• No MongoDB path collision occurs
• All existing projection behavior remains unchanged

[Himanshu197200]

@vkarpov15 Plz review this PR

[Himanshu197200]

@vkarpov15 I made the changes according to the error , please review it.

[Himanshu197200]

Hi @vkarpov15
I’ve made all the requested changes and updated the PR accordingly.
When you get a moment, could you please take another look and let me know if anything else is needed from my side?
Thanks for your time and guidance!