How to get collection back after insert/update in mongodb?

Try some thing:

For insert=>

public async Task<TEntity> AddAsync(TEntity entity)
{
    var doc = SetId(entity);
    await mongoDB.GetCollection<BsonDocument>(COLLECTION_NAME).InsertOneAsync(doc);
    return entity;
}

private BsonDocument SetId(TEntity entity)
{
    var bsonDoc = entity.ToBsonDocument();
    bsonDoc["_id"] = mongoDB.GetUniqueMongoDatabaseIDForRecord();
    return bsonDoc;
}

For update=>

public async Task<TEntity> UpdateAsync(TEntity entity)
{
    var id = GetId(entity);
    FilterDefinition<TEntity> filter = Builders<TEntity>.Filter.Eq("_id", id);
    await mongoDB.GetCollection<TEntity>(COLLECTION_NAME).ReplaceOneAsync(filter, entity);
    return entity;
}

private BsonValue GetId(TEntity entity)
{
    var bsonDoc = entity.ToBsonDocument();
    return bsonDoc.GetElement("_id").Value;
}