Marshal and umarshal BSON - mongodb

Question

TL;DR: Does the MongoDB driver provide a function to marshal and unmarshal a single field of a document?
This is a pretty straightforward question, but here's some context:
I have a worker responsible for synchronizing data between 2 separated databases. When it receives an event message, signalizing some document must sync, it selects the document in the primary database, and replicates it in another (it's a whole different database, not a replica set).
The thing is: I don't know the full structure of that document, so to preserve the data, I must unmarshal this document in a map map[string]interface{}, or a bson.M that works in the same fashion. But this seems like a lot of overhead, to unmarshal all this data I'm not even using, only to marshal it back to the other database.
So I thought about creating a structure that would just store the binary value of that document, without performing any marshal or unmarshal in order to reduce the overhead, like this:
type Document = map[string]Field
type Field struct {
Type bsontype.Type
Value []byte
}
func (f Field) MarshalBSONValue() (bsontype.Type, []byte, error) {
return f.Type, f.Value, nil
}
func (f *Field) UnmarshalBSONValue(btype bsontype.Type, value []byte) error {
f.Type = btype
f.Value = value
return nil
}
With this structure I can indeed reduce how much of the data will be parsed, but now, I need to manually unmarshal the one value in this document I'll need to use.
So I'm wondering if the MongoDB driver would have some function such as:
// Hypothetical function to get the value of a BSON
var status string
if err := decodeBSON(doc['status'].Type, doc['status'].Value, &status); err != nil {
return err
}
And
// Hypothetical function to set the value of a BSON
createdAt, err := encodeBSON(bsontype.Date, time.Now())
if err != nil {
return err
}
doc["createdAt"] = Field{Type: bsontype.Date, Value: createdAt}
How can I achieve this?

Answer 1

The Field type in your code is equivalent to the driver's bson.RawValue type. By switching to RawValue, you can decode individual fields using the RawValue.Unmarshal method and encode fields using bson.MarshalValue, which returns the two components (type and data) that you need to construct a new RawValue.
An example of how you can use these methods to change a field based on its original value: The Field type in your code is equivalent to the driver's bson.RawValue type. By switching to RawValue, you can decode individual fields using the RawValue.Unmarshal method and encode fields using bson.MarshalValue, which returns the two components (type and data) that you need to construct a new RawValue.
An example of how you can change a field depending on its original value without unmarshalling all of the original document's fields: https://gist.github.com/divjotarora/06c5188138456070cee26024f223b3ee