Update data type of field for all document using MongoDB C# driver

Question

I'm trying to update the data type of a field from string to ObjectId for all documents in a collection using MongoDB C# driver. This query works perfectly fine:

db.getCollection('MyCollection').updateMany(
    { MyField: { $type: 2 } },
    [{ $set: { MyField: { $toObjectId: "$MyField" } } }]
);

But I'm struggling to write the same query in C#. I've tried the following query using UpdateManyAsync:

var filter = new BsonDocument("MyField", new BsonDocument("$type", BsonType.String));
                var update = new BsonDocument {
                    { "$set", new BsonDocument("MyField", new BsonDocument("$toObjectId", "$MyField")) }
                };
var updateResult = await collection.UpdateManyAsync(filter, update);

But got the following error:

The dollar ($) prefixed field '$toObjectId' in 'MyField.$toObjectId' is not valid for storage

This example here works, but it's not ideal because it forces me to fetch all documents:

var updateList = new List<WriteModel<BsonDocument>>();
var documents = await collection.Find(Builders<BsonDocument>.Filter.Empty).ToListAsync();

foreach (var document in documents)
{
    var id = document.GetElement("_id").Value.AsString;
    var myFieldValue = document.GetElement("MyField").Value.AsString;

    var filter = Builders<BsonDocument>.Filter.Eq("_id", id);
    var update = Builders<BsonDocument>.Update.Set("MyField", new BsonObjectId(ObjectId.Parse(myFieldValue)));
    updateList.Add(new UpdateOneModel<BsonDocument>(filter, update));
}

if (updateList.Any())
{
    var bulkWriteResult = await collection.BulkWriteAsync(updateList);          
}

As understand from the first code and second. You have $toObjectId in first one and $toString in the next one. Maybe it causes an error? — kanils_
– kanils_, Commented Dec 29, 2021 at 15:54
Oops, that is a typo. I'm going to edit the question to fix that. — ncve
– ncve, Commented Dec 29, 2021 at 16:01
Found some info, could be helpful actually. mongodb.com/community/forums/t/… — kanils_
– kanils_, Commented Dec 29, 2021 at 17:28
Thank you @kanils_! You pointed me in the right direction! Check my answer below, please. :) — ncve
– ncve, Commented Dec 29, 2021 at 18:00

dododo · Accepted Answer · 2021-12-29 17:47:38Z

2

Check this:

        var pipeline = Builders<BsonDocument>
            .Update
            .Pipeline(new EmptyPipelineDefinition<BsonDocument>()
                .AppendStage<BsonDocument, BsonDocument, BsonDocument>("{ $set: { MyField: { $toObjectId: '$MyField' } } }"));
        coll.UpdateMany(
            new BsonDocument("MyField", new BsonDocument("$type", BsonType.String)),
            pipeline);

answered Dec 29, 2021 at 17:47

dododo

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ncve · Accepted Answer · 2021-12-29 17:59:07Z

1

This code example works:

var filter = new BsonDocument("MyField", new BsonDocument("$type", BsonType.String));
var stage = new BsonDocument { { "$set", new BsonDocument { { "MyField", new BsonDocument { { "$toObjectId", "$MyField" } } } } } };
var pipeline = PipelineDefinition<BsonDocument, BsonDocument>.Create(stage);
var update = Builders<BsonDocument>.Update.Pipeline(pipeline);

var result = await collection.UpdateManyAsync(filter, update);

Many thanks @kanils_ you pointed me in the right direction, this example Mongo Db driver C# aggregation for update also helped me to write the code above. Also many thanks @dododo for your suggestion.

answered Dec 29, 2021 at 17:59

ncve

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