Mongoose !: statics this undefined - mongodb

Question

import mongoose, { Schema, Document, Model } from "mongoose";
import { IUser } from "./interface/User.interface";
const userSchema = new Schema<IUserDoc>({
...
});
// Problem
userSchema.statics.findUser = async function (
email: Pick<IUser, "email">
): Promise<IUser | null> {
console.log("this", this); // undefined
try {
// Cannot read properties of undefined (reading 'findOne')
const user = await this.findOne({ email }).exec();
return user;
} catch (err: any) {
return err;
}
};
interface IUserDoc extends IUser, Document {
_id: string;
}
interface IUserModel extends Model<IUserDoc> {
findUser: (email: string) => Promise<IUser>;
}
const User = mongoose.model<IUserDoc, IUserModel>("User", userSchema);
export { User };
// Working
const findUserTest = async (email: string): Promise<IUser | null> => {
try {
const user = await User.findOne({ email }).exec();
return user;
} catch (err: any) {
return err;
}
};
mongoose -v 6.5.1
findUser does not work, but findUserTest works well. I think the problem is with userSchema, but I don't know what the problem is. I'd appreciate it if you could give me a hint.