fix(mongo-orm): .select() combined with .include() drops joined relation fields via $project stage

[rajat12826]

Fix MongoDB $project Dropping Included Relations

PR Description

Fixes #30169

Problem

When chaining .select() and .include() in a MongoDB ORM query, the query compiler emits a $lookup stage to join the related collection but then builds a $project stage containing only the selected scalar fields.

In MongoDB aggregation pipelines, an inclusion projection such as:

{ name: 1, _id: 0 }

drops all fields that are not explicitly listed. As a result, the relation field produced by $lookup is silently discarded at runtime, even though TypeScript infers it as present.

Example

const results = await db.user
  .select('name')
  .include('posts')
  .all();
// Expected:

// { name: "Alice", posts: [...] }
// Actual:

// { name: "Alice" } ← posts dropped by $project

Fix

In compileMongoQuery, when building the $project projection object, iterate over state.includes and add each inc.relationName to the projection alongside the selected scalar fields.

Before

if (state.selectedFields && state.selectedFields.length > 0) {

const projection: Record<string, 0 | 1> = {};

for (const field of state.selectedFields) {

projection[field] = 1;

}
if (!Object.hasOwn(projection, '_id')) {

projection['_id'] = 0;

}
stages.push(new MongoProjectStage(projection));

}

After

if (state.selectedFields && state.selectedFields.length > 0) {

const projection: Record<string, 0 | 1> = {};

for (const field of state.selectedFields) {

projection[field] = 1;

}

for (const inc of state.includes) {
projection[inc.relationName] = 1;
}

if (!Object.hasOwn(projection, '_id')) {

projection['_id'] = 0;

}
stages.push(new MongoProjectStage(projection));

}

This ensures that relation aliases generated by $lookup are preserved by the subsequent $project stage.

Tests Added

Three new test cases were added to compile.test.ts under select combined with include:

Test	Asserts
$project retains to-many relation alias	{ name: 1, posts: 1, _id: 0 }
$project retains to-one relation alias	{ title: 1, author: 1, _id: 0 }
$project retains _id when explicitly selected alongside includes	{ _id: 1, name: 1, posts: 1 }

Summary

The fix ensures that combining .select() with .include() in MongoDB queries preserves the included relations in the final aggregation result.

Behavior

.select() + .include()
        │
        ▼
     $lookup
        │
        ▼
     $project
        │
        ├── selected scalar fields
        └── included relation aliases
        │
        ▼
   Correct result

Summary by CodeRabbit

• Bug Fixes

  • Relations included alongside selected fields now remain available in query results.
  • Corrected _id handling when combining field selection with relation includes.

• Tests

  • Added coverage for to-one and to-many relations combined with selected fields.
  • Added verification for implicit and explicit _id selection behavior.

[CLAassistant]

All committers have signed the CLA.

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📥 Commits

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📒 Files selected for processing (2)

• packages/2-mongo-family/5-query-builders/orm/src/compile.ts
• packages/2-mongo-family/5-query-builders/orm/test/compile.test.ts

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📝 Walkthrough

Walkthrough

The MongoDB ORM compiler now retains included relation aliases in projections built from selected fields. Tests cover to-many and to-one relations, including implicit and explicit _id behavior.

Changes

Projection relation retention

Layer / File(s)	Summary
Projection compiler and coverage packages/2-mongo-family/5-query-builders/orm/src/compile.ts, packages/2-mongo-family/5-query-builders/orm/test/compile.test.ts	The compiler adds each included relation alias to the projection. Tests cover to-many and to-one includes, implicit _id suppression, and explicit _id selection.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to 3ec07

The change preserves included relation fields when scalar selection is combined with relation inclusion. It is localized and covered by targeted tests, so no actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: aqrln

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically describes the main fix: preserving joined relation fields when MongoDB ORM queries combine .select() and .include().
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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