How can i build mongoose model from this JSON

Your comment schema is a bit confusing. You created a "Comment" schema (note this is singular) yet you are trying to use the comment schema as an array of comments.

Tip: Keep your schemas and singular.

To achieve your desired design whilst keeping your models singular you can try this:

Solution (1): Have a single "Section" collection storing all the dataNote: although you have multiple schemas, we only ever model the SectionSchema and it will be our only collection where every Section document contains all the info you need.

const sectionSchema = new Schema({    name: {        type: String,        required: [true, 'Section name is required']    },    comments: {        type: [commentSchema]    }});const replySchema = new Schema({    //id: String, //not needed, mongoose will automatically add an "_id" property when you save     authorAvatarUrl: String,    authorName: String,    authorId: String,    authorUrl: String,    comment: String,    parentId: String});const commentSchema = new Schema({    //id: String, //not needed, mongoose will automatically add an "_id" property when you save     authorAvatarUrl: String,    authorName: String,    authorId: String,    authorUrl: String,    comment: String,    replies:{        type: [replySchema]    }});module.exports =  mongoose.model("Section", sectionSchema);

In the above solution, you can still have routes to address only specific comments, for example:

router.get("/sections/:id") //gets the entire section document matching the IDrouter.post("/sections/:id/comments/") //creates a new comment within section with matching ID. Simply push a new comment into the comments arrayrouter.get("/sections/:id/comments/") //gets all the comments for this section. Simply return only the comments property of the section document.router.get("/sections/:id/comments/:commentId") //gets a specific comment within a specific section. Filter the comments array to get the comment matching the commentId and then only return that.etc..

Final note: there are other ways you can model your data. This is just one example. For instance, you can have a section collection and a comment collection, where a comment document stores a section Id indicating the section that that comment document relates to.

Have a look at https://mongoosejs.com/docs/subdocs.html and https://docs.mongodb.com/manual/core/data-modeling-introduction/ it might help you to understand the different ways you can model your data and also how mongoose subdocuments work and how to use them.