Update MongoDB collection where field doesn't exist

Question

I want to update every record in a collection that doesn't contain a particular field.

The following query works using Mongo shell, but I am struggling to write this using the mongoDB.Driver in .Net.

db.Comments.update(
    { MemberRoleType: { $exists: false }},
    { $set: { MemberRoleType: 4 },
    { multi: true }
)

I've attempted various methods such as follows but can't achieve the result I'm looking for :

await comments.UpdateManyAsync(c => c.MemberRoleType == 0, Builders<Comment>.Update.Set(x => x.MemberRoleType, ContentRoleType.Online)));

Answer 1

I believe you are looking for exists

Builders<Comments>.Filter.Exists(x => x.MemberRoleType, false);

You should be able to use it like so

var filter = Builders<Comments>.Filter.Exists(x => x.MemberRoleType, false);
await comments.UpdateManyAsync(filter, Builders<Comment>.Update.Set(x => x.MemberRoleType, ContentRoleType.Online)));

Which in turn generates this query

{
        "q": {
            "MemberRoleType": {
                "$exists": false
            }
        },
        "u": {
            "$set": {
                "MemberRoleType": 4
            }
        },
        "multi": true
}