Post.js

const mongoose       = requier("mongoose")
const { ObjectId }   = mongoose.Schema
      
const postSchema = new mongoose.Schema({
   text: String,
   image: String,
   likes: [{type: ObjectId, ref: "user"}],
   comments: [
      {
         text: String,
         createAt: {type: Date,default: Date.now},
         postedBy: {type: ObjectId, ref: "user"}
      }
   ],
   postedBy: {type: ObjectId,ref: "user"},
   createAt: {
      type: Date,
      default: Date.now
   }
})

const autoPopulatePostedBy = function(next){
   this.populate("postedBy" , "_id name avatar");
   this.populate("comments.postedBy" , "_id name avatar");
   next();
}

postSchema
   .pre("findOne" , autoPopulatePostedBy)
   .pre("find" , autoPopulatePostedBy)

postSchema.index({ postedBy: 1, createdAt: 1 });

module.exports = mongoose.model("post",postSchema)

User.js

const mongoose       = requier("mongoose")
const { ObjectId }   = mongoose.Schema

const userSchema = new mongoose.Schema({
   email: String,
   name: String,
   avatar: {
      type: String,
      default: "/static/images/profile-image.png"
   },
   about: String,
   following: [{type: ObjectId,ref: "user"}],
   followers: [{type: ObjectId,ref: "user"}],
},
   // auto createAt vs updateAt field
   {timestamps: true}
)

const autoPopulateFollowingAndFollowers = function(next){
   this.populate("following" , "_id name user");
   this.populate("followers" , "_id name user");
   next();
}

userSchema.pre("findOne" , autoPopulateFollowingAndFollowers)

module.exports = mongoose.model("user", userSchema)