Cast to ObjectId failed for value (reference to files) - node.js

Question

As the title says.
For more context : I have a small script (launched by php via exec) that will search for a chapter with a given id to do some changes on the images (scramble them) that I didn't manage to do in php (I'm newbie working with files).
here are my schema
//instantiate mongoose-gridfs
var gridfs = require('mongoose-gridfs')({
collection:'files',
mongooseConnection: mongoose.connection
});
//Files = gridfs.model;
var FilesSchema = gridfs.schema;
FilesSchema.add({
_id: Schema.Types.ObjectId,
name: String,
mime: String,
filename: String,
type: String
});
Files = mongoose.model('Files', FilesSchema);
var pageSchema = Schema({
order: Number,
image: { type: Schema.Types.ObjectId, ref: 'Files' },
scrambled: Boolean
});
var ChapterSchema = Schema({
_id: Schema.Types.ObjectId,
pages: [pageSchema]
});
var Chapter = mongoose.model('Chapter', ChapterSchema, "Chapter");
When I do that
Chapter.findById(chapterId)
.populate('pages.image')
exec(...
It raises the CastError : { CastError: Cast to ObjectId failed for value "DBRef ...
The error object is long, so I don't paste it here
but here is a pastebin with the full error message/object
https://pastebin.com/8n54Dhzt
If I chan,ge the type of Image to Array I can access the properties I need after, but in that case I can't reset the images properly (it writes an array on db instead of a Ref)
My ref is in an embeddedDocument, like that
Chapter.pages[0].image
image is the property that have the ref.
I don't really understand why I have this error, I followed the docs for the refs/populate : http://mongoosejs.com/docs/populate.html
PS : A detail that can be important, The Reference is saved by DoctrineODM.
I use my node script to modify/encrypt the image, and then revert the encryption in js on my app.
PS2: sorry if a similar question is already posted and answered, there is so much post with the same error 'Cast to ObjectId failed for value' I admit I didn't read all of them, but those I read didn't help me.
EDIT : Update my code to use mongoose-gridfs Model instead of a different one + use subDocument instead of Array of object

Answer 1

Well that seems an error with chapterIdvalue. When you use collection.findById, the passed value must pass the ObjectId criteria which is
12-byte structure, the first 4 bytes of the ObjectId represent the
time in seconds since the UNIX epoch.
The next 3 bytes of the ObjectId represent the machine identifier.
The next 2 bytes of the ObjectId represent the process ID.
And the last 3 bytes of the ObjectId represent a random counter
value.
And if your passed id is not constructable to the pattern mentioned above this is definitely gonna throw the errors like you're getting.
So make sure you chapterId is correct or it is the same what you are getting from the database