From 16bec60ea0b52aa5a55b810a2f1ef3df2a51a1df Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Mon, 1 Aug 2022 22:19:53 -0400
Subject: [PATCH] fix(model+query): handle populate with lean transform that
 deletes `_id`

Fix #12143
---
 lib/model.js       | 19 +++++++++++++++++++
 test/query.test.js | 38 ++++++++++++++++++++++++++++++++++++--
 2 files changed, 55 insertions(+), 2 deletions(-)

diff --git a/lib/model.js b/lib/model.js
index bbbf4febf43..b6633f9d2fc 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -4599,6 +4599,17 @@ function populate(model, docs, options, callback) {
       mod.options.options.sort || void 0;
     assignmentOpts.excludeId = excludeIdReg.test(select) || (select && select._id === 0);
 
+    // Lean transform may delete `_id`, which would cause assignment
+    // to fail. So delay running lean transform until _after_
+    // `_assign()`
+    if (mod.options &&
+        mod.options.options &&
+        mod.options.options.lean &&
+        mod.options.options.lean.transform) {
+      mod.options.options._leanTransform = mod.options.options.lean.transform;
+      mod.options.options.lean = true;
+    }
+
     if (ids.length === 0 || ids.every(utils.isNullOrUndefined)) {
       // Ensure that we set to 0 or empty array even
       // if we don't actually execute a query to make sure there's a value
@@ -4673,6 +4684,14 @@ function populate(model, docs, options, callback) {
     for (const arr of params) {
       removeDeselectedForeignField(arr[0].foreignField, arr[0].options, vals);
     }
+    for (const arr of params) {
+      const mod = arr[0];
+      if (mod.options && mod.options.options && mod.options.options._leanTransform) {
+        for (const doc of vals) {
+          mod.options.options._leanTransform(doc);
+        }
+      }
+    }
     callback();
   }
 }
diff --git a/test/query.test.js b/test/query.test.js
index 1ebc5320aa4..4387c336642 100644
--- a/test/query.test.js
+++ b/test/query.test.js
@@ -3992,7 +3992,7 @@ describe('Query', function() {
     });
   });
 
-  it('allows a transform option for lean on a query gh-10423', async function() {
+  it('allows a transform option for lean on a query (gh-10423)', async function() {
     const arraySchema = new mongoose.Schema({
       sub: String
     });
@@ -4004,7 +4004,7 @@ describe('Query', function() {
       foo: [arraySchema],
       otherName: subDoc
     });
-    const Test = db.model('gh10423', testSchema);
+    const Test = db.model('Test', testSchema);
     await Test.create({ name: 'foo', foo: [{ sub: 'Test' }, { sub: 'Testerson' }], otherName: { nickName: 'Bar' } });
 
     const result = await Test.find().lean({
@@ -4030,6 +4030,40 @@ describe('Query', function() {
     assert.strictEqual(single.foo[0]._id, undefined);
   });
 
+  it('handles a lean transform that deletes _id with populate (gh-12143) (gh-10423)', async function() {
+    const testSchema = Schema({
+      name: String,
+      user: {
+        type: mongoose.Types.ObjectId,
+        ref: 'User'
+      }
+    });
+
+    const userSchema = Schema({
+      name: String
+    });
+
+    const Test = db.model('Test', testSchema);
+    const User = db.model('User', userSchema);
+
+    const user = await User.create({ name: 'John Smith' });
+    let test = await Test.create({ name: 'test', user });
+
+    test = await Test.findById(test).populate('user').lean({
+      transform: (doc) => {
+        delete doc._id;
+        delete doc.__v;
+        return doc;
+      }
+    });
+
+    assert.ok(test);
+    assert.deepStrictEqual(test, {
+      name: 'test',
+      user: { name: 'John Smith' }
+    });
+  });
+
   it('skips applying default projections over slice projections (gh-11940)', async function() {
     const commentSchema = new mongoose.Schema({
       comment: String