Mongo C# driver update a specific element in a nested array

Question

How do I modify in Mongo (C# driver) a single element in a nested property (array) without retrieving the whole document?

public class Element
{
    public int Value {get; set;}

    public string Name {get; set;}
}

public class Document
{

     public Element [] Elements {get; set;}
}

In example I want to find the element with name "Car" and sets its value to 4 in a single query.

Josh Brown · Accepted Answer · 2023-03-31 05:51:21Z

10

You need $ positional operator where you can specify document-level condition and array-level condition to find single nested item in an array of particular document. In C# $ sign is represented by -1 passed as an index of your model array. Try:

var col = mydb.GetCollection<Document>("collectionName");
var id = new ObjectId("5babaaf5509f6d342da5abaa");
var elementName = "Car";
var newValue = 2;

var filterBuilder = Builders<Document>.Filter;
var filter = filterBuilder.Eq(x => x.Id, id) &
    filterBuilder.ElemMatch(doc => doc.Elements, el => el.Name == elementName);

var updateBuilder = Builders<Document>.Update;
var update = updateBuilder.Set(doc => doc.Elements[-1].Value, newValue);

Col.UpdateOne(filter, update);

Update: -1 is not supported in MongoDB.Driver 2.19 in .NET 6, you'll get Expression not supported: x.Elements.get_Item(-1) because negative indexes are not valid. To use the positional operator $ use FirstMatchingElement instead of an index value of -1. Just include that using MongoDB.Driver.Linq statement and replace the line above.

using MongoDB.Driver.Linq;

var update = updateBuilder.Set(doc => doc.Elements.FirstMatchingElement().Value, newValue);

edited Mar 31, 2023 at 5:51

Josh Brown

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answered Sep 26, 2018 at 15:58

mickl

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5 Comments

Hammad S. Over a year ago

I need to update all elements where matches the filter. It will only update one element?

mickl Over a year ago

@HammadSajid yes, this will update only one, for multiple you can try: stackoverflow.com/questions/60219952/…

Hammad S. Over a year ago

That's very sweet of you, I am searching for hours' but not able to found it

Peet vd Westhuizen Over a year ago

"In C# $ sign is represented by -1 passed as an index of your model array" Learned this here, thank you!

Dorian Farrimond Over a year ago

So grateful for the update above, which solved my problem after hours of searching around the web. The official documentation really needs some love. FirstMatchingElement is exactly what I needed, and nice and clean too.

tmaurst · Accepted Answer · 2024-03-05 07:53:57Z

For those looking for a generic method, this works as of driver version 2.23.1. T is the parent doc and C is the sub/nested doc.

public async Task UpdateArrayItemAsync<C>(
    Expression<Func<T, bool>> filter, // eg. (parent => parent.Id == "1")
    Expression<Func<C, bool>> arrayFilter, // eg. (sub => sub.Id == "1a")
    string arrayField, // Name of the array field on the parent doc
    IDictionary<string, object> updates) // Fields and values to set on array object
{
    var updateFilter = Builders<T>.Filter.And(
        Builders<T>.Filter.Where(filter),
        Builders<T>.Filter.ElemMatch<C>(arrayField, arrayFilter));
    
    // '.$' refers to the first matched element in Builders<T>.Filter.ElemMatch<C>()
    var updateDefs = updates.Keys
        .Select(field => Builders<T>.Update.Set(
            $"{arrayField}.$.{field}", updates[field]));

    await _mongoCollection.UpdateOneAsync(updateFilter,
        Builders<T>.Update.Combine(updateDefs));
}

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