Update with aggregation pipeline in C# .NET

Using NuGet MongoDB.Driver version 2.14.0, I’m trying to perform update of document - copying one field (primary) to another (secondary) and then updating copied field value.

Document looks like this:

  _id: ...,
  primary: {
    /* fields */
    updatedAt: "2022-03-24T00:00:00.000Z"
  secondary: {
    /* fields */
    updatedAt: "2022-01-01T00:00:00.000Z"

This is MongoDB query which does what I want:

  { _id: ... }, 
    /* copy `primary` to `secondary` */
    { $set: { secondary: "$primary" } },

    /* update `updatedAt` at `secondary` with current time */
    { $set: { "secondary.updatedAt": "2022-03-24T00:00:01.000Z" } }

(… at least I hope this query will work as expected, not sure whether I should worry about race condition)

Now I want to achieve this from .NET (5), but I can’t find any article, example nor unit test demonstrating how to do it.

I have vague feeling I need to use EmptyPipelineDefinition<> but I can’t figure out how; it is also preferable to use my entity avoiding writing BsonDocument manually - which could be error prone (having field names in code in strings).

Could anyone point me to proper place where I can get more details?

Hi, @Zdenek_Havlin,

Welcome to the MongoDB Community Forums.

I understand that you are trying to compose an update operation that involves setting one field in a document to the value of another field in the same document. In C#, this would look like:

var update = Builders<Article>.Update.Set(x => x.Secondary, x => x.Primary); // will not compile

While this is possible in MongoDB update operations, we have not implemented support yet for this in the .NET/C# driver. See CSHARP-3677 for this feature request.

In the meantime, you can implement this using EmptyPipelineDefinition<> as you suspected. After creating an EmptyPipelineDefinition<>, then call AppendStage stage for each update operation that you want to perform. You can specify these stages using BsonDocument objects or a string containing valid JSON. Note that I had to use Extended JSON v1 Syntax to ensure that the date value was not converted to a string during the update operation.

using System;
using MongoDB.Bson;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;

var client = new MongoClient();
var db = client.GetDatabase("test");
var coll = db.GetCollection<Article>("articles");

var filter = Builders<Article>.Filter.Empty; // actual filter goes here
var pipeline = new EmptyPipelineDefinition<Article>()
    .AppendStage<Article, Article, Article>("{ $set: { secondary: '$primary' } }")
    .AppendStage<Article, Article, Article>($"{{ $set: {{ 'secondary.updatedAt': {{$date: '{DateTime.UtcNow}' }} }} }}");
var update = Builders<Article>.Update.Pipeline(pipeline);
coll.UpdateOne(filter, update);

class Article
    public ObjectId Id { get; set; }
    public Entry Primary { get; set; }
    public Entry Secondary { get; set; }

class Entry
    public DateTime UpdatedAt { get; set; }

Please let me know if you have any additional questions and be sure to watch/vote for CSHARP-3677 if it would be of use to you.


Hi @James_Kovacs ! Thanks, that worked like a charm :slight_smile: I was bit confused by that JSON string input - made it to BsonDocument in my code (it’s bit longer but probably not so controversial to my fellow .NET developers).

Once again, thank you for your help!

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