Mongoose populate() returns array of objects instead of single object
It makes sense since you in the schema definition have declared the category field to be an array:
category: [{ type: Schema.Types.ObjectId, ref: 'categories' }]The following should work to not put it in an array (remove the []):
category: { type: Schema.Types.ObjectId, ref: 'categories' }That is assuming that you indeed want only one category.