From 008d95dd6b13ac831427a5ffbe84b841efe4ec43 Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Mon, 17 Nov 2025 13:15:50 -0500
Subject: [PATCH 1/2] types: correct Model.schema type and fix unknown check
 for `this` param type in schema.methods

Fix #15693
---
 test/types/models.test.ts | 28 ++++++++++++++++++++++++++++
 types/models.d.ts         |  2 +-
 types/utility.d.ts        |  3 ++-
 3 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/test/types/models.test.ts b/test/types/models.test.ts
index 22b09169412..0cd4b08c1eb 100644
--- a/test/types/models.test.ts
+++ b/test/types/models.test.ts
@@ -1056,3 +1056,31 @@ async function gh16526() {
   const insertManyResult = await Tank.insertMany([{ name: 'test' }], { lean: true, rawResult: true });
   expectType<number>(insertManyResult.insertedCount);
 }
+
+async function gh15693() {
+  interface IUser {
+    name: string;
+  }
+
+  interface UserMethods {
+    printName(this: IUser): void;
+    getName(): string;
+  }
+
+  const schema = new Schema<IUser, Model<IUser>, UserMethods>({ name: { type: String, required: true } });
+  schema.method('printName', function printName(this: IUser) {
+    expectError(this.isModified('name'));
+    expectError(this.doesNotExist());
+    expectType<string>(this.name);
+    console.log(this.name);
+  });
+  schema.method('getName', function getName() {
+    expectType<boolean>(this.isModified('name'));
+    return this.name;
+  });
+  const User = model('user', schema);
+
+  const leanInst = await User.findOne({}).lean().orFail();
+  User.schema.methods.printName.apply(leanInst);
+
+}
diff --git a/types/models.d.ts b/types/models.d.ts
index afadaff43e4..b2ad98c627f 100644
--- a/types/models.d.ts
+++ b/types/models.d.ts
@@ -1081,7 +1081,7 @@ declare module 'mongoose' {
     recompileSchema(): void;
 
     /** Schema the model uses. */
-    schema: Schema<TRawDocType>;
+    schema: TSchema;
 
     /** Creates a `updateMany` query: updates all documents that match `filter` with `update`. */
     updateMany<ResultDoc = THydratedDocumentType>(
diff --git a/types/utility.d.ts b/types/utility.d.ts
index 44d5ac1b0cd..33ac1e176db 100644
--- a/types/utility.d.ts
+++ b/types/utility.d.ts
@@ -3,6 +3,7 @@ declare module 'mongoose' {
     ? THENTYPE
     : ELSETYPE;
   type IfUnknown<IFTYPE, THENTYPE> = unknown extends IFTYPE ? THENTYPE : IFTYPE;
+  type IsUnknown<T> = unknown extends T ? true : false;
 
   type WithLevel1NestedPaths<T, K extends keyof T = keyof T> = {
     [P in K | NestedPaths<Required<T>, K>]: P extends K
@@ -135,7 +136,7 @@ declare module 'mongoose' {
    */
   type AddThisParameter<T, D> = {
     [K in keyof T]: T[K] extends (...args: infer A) => infer R
-      ? ThisParameter<T[K], unknown> extends unknown
+      ? IsUnknown<ThisParameter<T[K], unknown>> extends true
         ? (this: D, ...args: A) => R
         : T[K]
       : T[K];

From c2dc34d03dfab764d6ccb46773f1e25b644adbcb Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Mon, 17 Nov 2025 13:20:47 -0500
Subject: [PATCH 2/2] types: remove IfUnknown, no longer used

---
 types/utility.d.ts | 1 -
 1 file changed, 1 deletion(-)

diff --git a/types/utility.d.ts b/types/utility.d.ts
index 33ac1e176db..4c537065411 100644
--- a/types/utility.d.ts
+++ b/types/utility.d.ts
@@ -2,7 +2,6 @@ declare module 'mongoose' {
   type IfAny<IFTYPE, THENTYPE, ELSETYPE = IFTYPE> = 0 extends 1 & IFTYPE
     ? THENTYPE
     : ELSETYPE;
-  type IfUnknown<IFTYPE, THENTYPE> = unknown extends IFTYPE ? THENTYPE : IFTYPE;
   type IsUnknown<T> = unknown extends T ? true : false;
 
   type WithLevel1NestedPaths<T, K extends keyof T = keyof T> = {