How to query Firestore collection for documents that do not have a specific reference field using AngularFire?

Question

I have a Firestore collection named "operation" where each document may or may not have a field named "user" of type reference. I want to retrieve only the documents that do not have the "user" field. I'm using AngularFire in my Angular application. Is there a way to achieve this using AngularFire's query functions? Here's an example of what I'm trying to achieve:

getOperations(){
    let operations = this.fstore
                    .collection<Operation>('operation', 
                      x => x.where('user','==', /*WHAT TO PUT HERE*/)
                      .orderBy('created_at', 'desc')
                    )
                    .snapshotChanges();
    return operations;
}