Question 299353 in Mongodb Question posted in
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Delete and return document in nested array with mongodb c# driver

I’m having trouble deleting an document in an array using the offical mongodb c# driver. What i’m trying to do is also return the document that was deleted.

Here’s what my class looks like:

public class Holder
{
    public string Owner { get; set; }
    public List<Card> Cards { get; set; }
}

public class Card
{
    public string Id { get; set; }
    public string Name { get; set; }
    public string Placement { get; set; }
}

And here is an exerpt of the document:

{
  "owner": "656dd33e0300"
  "cards" : [
    {
      "id": "s39CNzu4Na3",
      "name" : "Department",
      "placement" : "8a"
    },
    ...
  ]
}

My first thought was to just get the card and then delete it using the following:

var filter = Builders<Card>.Filter.Eq(x => x.Id, cardId);

var res = await collection.DeleteOneAsync(
     Builders<Holder>.Filter.Eq(e => e.Owner, owner) &
     Builders<Holder>.Filter.ElemMatch(e => e.Cards, filter)
    );
try {            
    return res.DeletedCount > 0;
}
catch (Exception ex) {
    throw;
}

But that just ended up deleting all the cards in the list.
I’ve been trying to use FindOneAndDeleteAsync but i can’t get it to work.

Tags:

2

Answers


  1. 0

    From your code, you are deleting the entire Holder document but not removing the Card element from the Cards array in the Holder document.

    I don’t think the MongoDB .NET driver supports removing and returning the deleted element (API). Thus, you should:

    1. Filter the document and get the filtered Card elements.

    2. Pull the Card element(s) which is matched from the Cards array.

    3. Return res.ModifiedCount > 0 indicates the card element is removed and the items removed.

    var filter = Builders<Card>.Filter.Eq(x => x.Id, cardId);

var holder = await collection.Find(
        Builders<Holder>.Filter.Eq(e => e.Owner, owner) &
        Builders<Holder>.Filter.ElemMatch(e => e.Cards, filter))
    .Project<Holder>(Builders<Holder>.Projection
        .Include(x => x.Owner)
        .ElemMatch(x => x.Cards, filter))
    .FirstOrDefaultAsync();

if (holder == null || !holder.Cards.Any())
{
    return new
    {
        IsCardDeleted = false,
        DeletedCards = (List<Card>)null
    };
}

UpdateResult res = await collection.UpdateOneAsync(
     Builders<Holder>.Filter.Eq(e => e.Owner, owner) &
     Builders<Holder>.Filter.ElemMatch(e => e.Cards, filter),
     Builders<Holder>.Update.PullFilter(e => e.Cards, filter)
);

if (res.ModifiedCount == 0)
{
    return new
    {
        IsCardDeleted = false,
        DeletedCards = (List<Card>)null
    };
}

return new
{
    IsCardDeleted = true,
    DeletedCards = holder.Cards
};

    enter image description here

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  2. 0

    As far as I understand your question, you do not want to delete the root document, but remove a sub-document from an array. You can do this using an update-Statement (delete is for deleting root documents as a whole).

    In order for this to work, you need to create a filter that finds the document and then run an update with a PullFilter on the document.

    In addition, you want to return the sub-document that you deleted during the update. There is no statement to return the sub-document, but you can use FindOneAndUpdateAsync to return the root document in the state before the sub-document was removed:

    var arrayFilter = Builders<Card>.Filter.Eq(x => x.Id, cardId);
// Find a document with a matching owner that contains at least one card with the specified id
var docFilter = Builders<Holder>.Filter.Eq(e => e.Owner, owner) 
    & Builders<Holder>.Filter.ElemMatch(e => e.Cards, arrayFilter);
// Remove all cards from the array that have a matching id 
// (id should be unique so only one card should be removed)
var update = Builders<Holder>.Update.PullFilter(e => e.Cards, arrayFilter);
// Find the document and return it in the state before the update
var before = await coll.FindOneAndUpdateAsync(
    docFilter,
    update,
    new FindOneAndUpdateOptions<Holder, Holder>() { ReturnDocument = ReturnDocument.Before });
// Get removed card from the returned document
var removedCard = before.Cards.SingleOrDefault(x => x.Id == cardId);
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