I return the Document _id and use it as a req.params.id to get the document at a later stage.
ObjectId.isValid() returns true, then using it in either ObjectId() or Find or FindOne will result in a cast error or
hex is not a function.
Basically also copy pasted the _id straight from the database to test.
I'm using mongoose ^4.7.6; _id looks like: 586e30a597f85b69891df304
[Stack trace]
[CastError: Cast to ObjectId failed for value "586e30a597f85b69891df304" at path "_id" for model "Module"] message: 'Cast to ObjectId failed for value "586e30a597f85b69891df304" at path "_id" for model "Module"', name: 'CastError', stringValue: '"586e30a597f85b69891df304"', kind: 'ObjectId', value: '586e30a597f85b69891df304', path: '_id', reason: undefined,
this is a bug in Mongoose >=4.7.3 https://github.com/Automattic/mongoose/issues/4867
bson package is really buggy
Turns out it's a bug in that version of mongoose. Removing my node_modules and reinstalling with mongoose set to 4.3.3 fixed this issue.