MongoDB C# Driver Update Collection with Concatenated string - mongodb

Question

How do I convert this SQL to MongoDB query using C# Driver
UPDATE dbo.MyTable SET ConcatField = CONCAT(Field1, Field2, Field3, Field4, Field5)
WHERE Id = 21
Using MongoDB.Driver 2.2.3.3
I need MongoDB query using BsonDocument type, I don't have Strong types for my Mongo Collections as Collection is not based on fixed schema.
Trying something like this
var items = myCollection.FindSync(filter).ToList();
foreach (var item in items)
{
UpdateDefinition<BsonDocument> updateDefinition =
new BsonDocumentUpdateDefinition<BsonDocument>(item.Merge(ListOfStrinForSelectedFields.ToBsonDocument()));
myCollection.UpdateManyAsync(filter, updateDefinition);
}

Answer 1

This will be my Shell Script
var cursor = db.MyCollection.find({ "Id": 21 }), // Or what ever your find conditions is
bulkUpdateOps = [];
cursor.forEach(function(doc){
var ConcatField = doc.Field1 + doc.Field2 + doc.Field3 ;
bulkUpdateOps.push({
"updateOne": {
"filter": { "_id": doc._id },
"update": { "$set": { "MyConCatField": ConcatField } }
}
});
if (bulkUpdateOps.length == 1000) {
db.MyCollection.bulkWrite(bulkUpdateOps);
bulkUpdateOps = [];
}
});
if (bulkUpdateOps.length > 0) { db.MyCollection.bulkWrite(bulkUpdateOps); }
then execute it in c# with RunCommandAsync method from MongoDatabase.
var result = await mongoDatabase.RunCommandAsync<BsonDocument>(BsonDocument.Parse(command));
Note: you will have to modify the command string using pipelines and parse it to BsonDocument.