From b8d354fa3d0a6795d08fa18b3da351fb989c204e Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Fri, 27 Dec 2024 15:45:03 -0500
Subject: [PATCH 1/2] fix(schema): make duplicate index error a warning for now
 to prevent blocking upgrading

Fix #15109
Re: #15112
---
 lib/schema.js       |  2 +-
 test/schema.test.js | 55 ++++++++++++++++++++++++++++-----------------
 2 files changed, 36 insertions(+), 21 deletions(-)

diff --git a/lib/schema.js b/lib/schema.js
index ddde465929..9f27e2ce23 100644
--- a/lib/schema.js
+++ b/lib/schema.js
@@ -2148,7 +2148,7 @@ Schema.prototype.index = function(fields, options) {
 
   for (const existingIndex of this.indexes()) {
     if (options.name == null && existingIndex[1].name == null && isIndexSpecEqual(existingIndex[0], fields)) {
-      throw new MongooseError(`Schema already has an index on ${JSON.stringify(fields)}`);
+      utils.warn(`Duplicate schema index on ${JSON.stringify(fields)} found. This is often due to declaring an index using both "index: true" and "schema.index()". Please remove the duplicate index definition.`);
     }
   }
 
diff --git a/test/schema.test.js b/test/schema.test.js
index e7c8a720cb..a5ad43360e 100644
--- a/test/schema.test.js
+++ b/test/schema.test.js
@@ -19,6 +19,7 @@ const DocumentObjectId = mongoose.Types.ObjectId;
 const vm = require('vm');
 const idGetter = require('../lib/helpers/schema/idGetter');
 const applyPlugins = require('../lib/helpers/schema/applyPlugins');
+const utils = require('../lib/utils');
 
 /**
  * Test Document constructor.
@@ -3279,29 +3280,43 @@ describe('schema', function() {
     assert.equal(subdoc.getAnswer(), 42);
   });
   it('throws "already has an index" error if duplicate index definition (gh-15056)', function() {
-    const ObjectKeySchema = new mongoose.Schema({
-      key: {
-        type: String,
-        required: true,
-        unique: true
-      },
-      type: {
-        type: String,
-        required: false
-      }
-    });
+    sinon.stub(utils, 'warn').callsFake(() => {});
+    try {
+      const ObjectKeySchema = new mongoose.Schema({
+        key: {
+          type: String,
+          required: true,
+          unique: true
+        },
+        type: {
+          type: String,
+          required: false
+        }
+      });
 
-    assert.throws(() => {
       ObjectKeySchema.index({ key: 1 });
-    }, /MongooseError.*already has an index/);
+      assert.equal(utils.warn.getCalls().length, 1);
+      let [message] = utils.warn.getCalls()[0].args;
+      assert.equal(
+        message,
+        'Duplicate schema index on {"key":1} found. This is often due to declaring an index using both "index: true" and "schema.index()". Please remove the duplicate index definition.'
+      );
 
-    ObjectKeySchema.index({ key: 1, type: 1 });
-    assert.throws(() => {
       ObjectKeySchema.index({ key: 1, type: 1 });
-    }, /MongooseError.*already has an index/);
-
-    ObjectKeySchema.index({ type: 1, key: 1 });
-    ObjectKeySchema.index({ key: 1, type: -1 });
-    ObjectKeySchema.index({ key: 1, type: 1 }, { unique: true, name: 'special index' });
+      assert.equal(utils.warn.getCalls().length, 1);
+      ObjectKeySchema.index({ key: 1, type: 1 });
+      assert.equal(utils.warn.getCalls().length, 2);
+      [message] = utils.warn.getCalls()[1].args;
+      assert.equal(
+        message,
+        'Duplicate schema index on {"key":1,"type":1} found. This is often due to declaring an index using both "index: true" and "schema.index()". Please remove the duplicate index definition.'
+      );
+
+      ObjectKeySchema.index({ type: 1, key: 1 });
+      ObjectKeySchema.index({ key: 1, type: -1 });
+      ObjectKeySchema.index({ key: 1, type: 1 }, { unique: true, name: 'special index' });
+    } finally {
+      sinon.restore();
+    }
   });
 });

From a5e4e61f76b9c90ac3ac005fb55f5e944e1ed94d Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Mon, 30 Dec 2024 15:08:04 -0500
Subject: [PATCH 2/2] test: add missing assertion to assert no further warnings

---
 test/schema.test.js | 1 +
 1 file changed, 1 insertion(+)

diff --git a/test/schema.test.js b/test/schema.test.js
index a5ad43360e..b8f4daf21a 100644
--- a/test/schema.test.js
+++ b/test/schema.test.js
@@ -3315,6 +3315,7 @@ describe('schema', function() {
       ObjectKeySchema.index({ type: 1, key: 1 });
       ObjectKeySchema.index({ key: 1, type: -1 });
       ObjectKeySchema.index({ key: 1, type: 1 }, { unique: true, name: 'special index' });
+      assert.equal(utils.warn.getCalls().length, 2);
     } finally {
       sinon.restore();
     }