如果我在Firebase中有两个集合：

Invoices:
    invoice1 {
        Vendor: (reference) /Vendors/vendor1
    }

Vendors:
    vendor1 {
        Name: "John Doe"
}

如果我用Cloud Firestore ODM生成器将它们建模为：

@Collection<Invoice>('Invoices')
@firestoreSerializable
class Invoice {
  Invoice({
      required this.id,
      required this.vendor,
  });

    @Id()
    String id;
    @JsonKey(name: 'Vendor')
    DocumentReference<Map<String, dynamic>> vendor;
}

final invoicesRef = InvoiceCollectionReference();

@Collection<Vendor>('Vendors')
@firestoreSerializable
class Vendor {
    Vendor({
      required this.id,
      required this.name,
  });

    @Id()
    String id;
    @JsonKey(name: 'Name')
    String name;
}

final vendorsRef = VendorCollectionReference();

当我从vendorsRef中选择一个供应商时，我会获得一个VendorDocumentReference。为了创建一个新的发票，我需要将它转换为DocumentReference<Map<String，dynamic>>。如何实现这种转换？
VendorDocumentReference或DocumentReference似乎没有执行此转换的方法。