is mongoose generated sub-document _id unique

Question

Using the following mongoose schema

mongoose.Schema({
    world: String,
    color: [{ name: String }]
});

Gives me document that have sub-documents containing _id fields.

{ _id: 'a9ec8475bf0d285e10ca8d42'
  world: 'matrix', 
  color: [
    { name: 'blue',  _id: '4a8c0e12135fa32e13db9ce9' },
    { name: 'red',   _id: '4a8c0a62254cd32e13db4ad8' },
    { name: 'white', _id: '4a8c04e2687ea32e13db1da7' }
]

I want to know if each of these sub-documents _id are unique across all documents, or only across the nested level it is in.

Edit : the sub-document doesn't refer to another collection, it's created with the plain mongoose schema above.

Answer 1

I assume the sub-document references another collection. If my assumption is right then the nested documents you have are only unique only on the nested level.

Answer 2

This is an issue with Mongoose. Mongoose creates schemas for each of those nested objects behind the scenes. That means the _id is only unquie on the nested level.

Look at this previous question on how to prevent this from happening