export interface User extends Document {
  email: string;
  password: string;
}

export const UserSchema: Schema = new Schema({
...
});

UserSchema.methods.checkPassword = function(...){...};

async findOneByEmail(email: string): Promise<User | null> {
    const userModel = await this.userModel.findOne({ email }).exec();
    return userModel;
  }

const user = await this.usersService.findOneByEmail(email);
if (!user) throw new UnauthorizedException();
userToAttempt.checkPassword(...

export interface User extends Document {
  email: string;
  password: string;

  checkPassword(...): any;
}