MongoDB updateMany set a boolean field using equals

Question

I have an IsDefault field in my model, and I want to set it to true for one document only and false for the others. Is this possible in a single query? I tried

{ $set: { IsDefault: {$eq: [_id, id ]}}}
 
{ $set: { IsDefault: { $cond: { if: { $eq: [$_id, id] }, then: true, else: false } } } }

and it didn't work. I also tried passing a lambda expression but it won't build either. Any idea if this is possible or do I need to use two seperate updateOne statements to the the current IsDefault to false, and the new one to true?

Answer 1

you can achieve it with an aggregation pipeline update like this:

db.collection.updateMany(
    {},
    [
        {
            $set: {
                IsDefault: {
                    $eq: ["$_id", ObjectId("5f65c2fcaf29d00898173d03")]
                }
            }
        }
    ])

here's a full example using mongodb.entities:

 using MongoDB.Entities; using System.Threading.Tasks; namespace StackOverflow { public class Book : Entity { public string Title { get; set; } public bool IsDefault { get; set; } } static class Program { private static async Task Main() { await DB.InitAsync("test", "localhost"); var books = new[] { new Book { Title = "book 1"}, new Book { Title = "book 2"}, new Book { Title = "book 3"} }; await books.SaveAsync(); var stage = new Template<Book>(@" { $set: { IsDefault: { $eq: ['$_id', ObjectId('<idvalue>')] } } }") .Tag("idvalue", books[1].ID); await DB.Update<Book>() .Match(_ => true) .WithPipelineStage(stage) .ExecutePipelineAsync(); } } }