How to save JSON array in to mongodb collection
It appears you declared features to be a String in your schema instead of an Array of subdocuments. Mongoose is therefore casting the array from req.body to a string before it is saved. The following tweak to your schema should get you on the right track:
var properties = new Schema({ Category_Id : { type: String}, Sub_category_Id : { type: String}, features : [{ specification_field_type": String, specification_name: String // any other paths for features }] });