Why do they use an ObjectId and a Number in the Mongoose Population example?
Type of reference has to be the same as the referenced schema's _id property.
In case of personSchema it's a Number.
storySchema on the other hand, has the _id field assigned automatically by mongoose - it's not specified in parameters for the schema constructor.
Mongoose assigns each of your schemas an _id field by default if one is not passed into the Schema constructor. The type assiged is an ObjectId to coincide with MongoDBs default behavior