TypeGraphQL Field Resolver with mongoose relationship

I have trouble dealing with populating the field using TypeGraphQL.

Situation summary with an example:

TypeGraphQL

TypeDef

@ObjectType()
class User {
  ...
}

@ObjectType()
class Post {
  ...

  @Field()
  user: User
}

Resolver

import { Post, User } from '@/models' // mongoose schema 

@Resolver(() => Post)
class PostResolver {
  @Query(() =>. Post)
  async getPost(id: string) {
    return await Post.findById(id);
  }

  ...
  
  @FieldReoslver(() => User) 
  async user(@Root() post: Post) {
    return await User.findById(post.user) // Error Occurs here.
  }
}

Mongoose

PostSchema

const PostSchema = new Schema({
  ...

  user: {
    type: Schema.ObjectId,
    ref: "User",
  }
})

I want to populate user field when the data Post is requested, with the field User type.
So, I used @FieldResolverlike above, but I encountered the Type Error because post.user is a type of User, not ObjectId of mongoose.

The user field is a type of ObjectId first when the getPost resolver was executed, but I want to populate this field to User when the client gets the response.

How can I get through this?

Thanks in advance.