mongodb c# combining set operations conditionally

Question

I am looking for a way to conditionally combine Set operations. At the moment I have been unable to increment onto the updatedefinitions without having them consecutively dotted one after the other.

eg. instead of:

    Builders<BsonDocument>.Update.Set("someElement.Length", b.Length)
.Set("someElement.Path", b.Path)

I am trying to get find a way to use something in the vain of:

var update = Builders<BsonDocument>.Update;
bool hasChanged = false;
if (a.Length != b.Length)
{
    hasChanged = true;
    update.Set("someElement.Length", b.Length)
}
if (a.Path != b.Path)
{
    hasChanged = true;
    update.Set("someElement.Path", b.Path)
}

if (hasChanged)
    await someCollection.UpdateOneAsync(Builders<someModel>.Filter.Eq("_id", a.Id), update);

Is there a way of doing this or am I chasing a pie in the sky? I dont want to replace the entire document, and am looking to only update fields that have changed.

Why not just contruct the BSON document for the update directly rather than using helpers? As a BSON document, the top level key is `"$set"`, and all fields for that operation would be children of that key. It's basically just a hash/map, so the same methods apply as for adding/constructing any of those. This is really all the "helper" functions like `Set()` are really doing anyway. — Blakes Seven, Mar 08 '16 at 10:47
Ok sounds interesting so it would be { $set: { someElement.length: B.length, someElement.path: b.path } } etc etc with the actual values instead. If so then your right. Thats alot easier and I can easily see how such functionality would be powerful. — xxdefaultxx, Mar 08 '16 at 12:21
I know the documentation follows the "shell" convention and therefore examples are in JSON, but if you basically think in terms that the eventual BSON looks pretty much the same ( in abstract terms ) then its's just a matter of construction. You would do a lot better to familarize with core documentation than driver documentation. See [`$set`](https://docs.mongodb.org/manual/reference/operator/update/set/). Of course I'm right. — Blakes Seven, Mar 08 '16 at 12:27
Thanks for the advice I wasn't questioning your accuracy only my interpretation. I'll def focus more on the core docs. Appreciated. Btw. If you can write in an example as an answer I can mark your answer as being the solution. — xxdefaultxx, Mar 08 '16 at 12:54

Craig Wilson · Answer 1 · 2016-03-10T16:43:39.617

14

UpdateDefinition is an immutable object, so chaining operations on them keeps creating a new one each time. To do it conditionally, you need assign the result back to itself, just like LINQ.

update = update.Set(...);

edited Mar 10 '16 at 16:43

answered Mar 08 '16 at 12:45

Craig Wilson

12,174
3
41
45

score 1 · Answer 2 · answered Sep 15 '20 at 14:32

If you maintain a collection of your conditionally created UpdateDefinitions, you can pass that collection into the Combine operator to create your final UpdateDefinition.

You can find an example on this similar question: C# MongoDB Driver - How to use UpdateDefinitionBuilder?

mongodb c# combining set operations conditionally

2 Answers2