Mongoose populate not working in my mern project

Working with Data Node.js Frameworks
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1

I am building a MERN stack project, this project have a feature social media. Like social media nowdays, you have a post, profile, and comment in the post. I tried to implementing populate for my data in the backend but its not working. What did i do wrong in the code, can someone please explain it to me.

it return empty array

My 3 model :

const CommentSchema = new mongoose.Schema({
   ContentMessage : {
          type : String
   },
   PostID : {
          type : String
   },
   WriterID : {
          type : String
   },
   user : {
       type : mongoose.Schema.Types.ObjectId,
       ref : "sosmedprofiles"
   },
   post : {
         type : mongoose.Schema.Types.ObjectId,
          ref : "sosmedposts"
   }
}, {timestamps : true});



const CommentModel = mongoose.model("comments", CommentSchema);
module.exports = CommentModel;

const SosmedPostSchema = new mongoose.Schema({
   Username : {
        type : String
   }, 
   Content : {
        type : String
   },
   Documents : {
        type : String
   },
   user : {
     type : mongoose.Schema.Types.ObjectId,
     ref : "sosmedprofiles"
   },
   comments : [{
     type: mongoose.Schema.Types.ObjectId,
     ref: "comments"
   }]
}, {timestamps : true});



const SosmedPostModel = mongoose.model("sosmedposts", SosmedPostSchema);
module.exports = SosmedPostModel;

const SosmedProfileSchema = new mongoose.Schema({
   Username : {
        type : String
   },
   FullName : {
        type : String
   },
   ProfilePicture : {
        type : String
   },
   Bio : {
        type : String
   },
   Post : [{
     type : mongoose.Schema.Types.ObjectId,
     ref : "sosmedposts"
   }]
}, {timestamps : true});


const SosmedProfileModel = mongoose.model("sosmedprofiles", SosmedProfileSchema);
module.exports = SosmedProfileModel

Post Route/Controller :

const getPostById = (req, res) => {
    const Id = req.params.id;

   SosmedPostModel.findOne({_id : Id})
   .populate({
      path : "user",
      populate : {
        path : "comments"
      }
   })
   .then(posts => {
    res.json(posts);
   })
}

router.get("/socialmedia/post/:id", getPostById);