How to remove a single Element from Array

Question

I want to remove one field from every document using MongoDB driver for C#.

I have documents like this:

{
  "_id": ObjectId("554e05dfc90d3d4dfcaa2aea"),
  "username": "abc",
  "someArray": [
    {
      "_id": ObjectId("554e0625a51586362c33c6df"),
      "IsReadOnly": false
    },
    {
      "_id": ObjectId("554e0625a51586362c33c6df"),
      "IsReadOnly": true
    }
  ]
} 

I want to remove the field "IsReadOnly" from "someArray", regardless of whether the value is true or false.

In MongoDB I can reach that with the following script:

db.getCollection('Collection').find({}).forEach( function(doc) {
  var arr = doc.someArray;
  var length = arr.length;
  for (var i = 0; i < length; i++) {
    delete arr[i]["IsReadOnly"];
  }
  db.getCollection('Collection').save(doc);
});

In C# I tried the following:

var update= Update.Pull("someArray",BsonValue.Create("IsReadOnly"));

// or: var update = Update.Unset("someArray.$[].IsReadOnly");
myDb.Collection.Update(null, update, UpdateFlags.Multi);

But the "IsReadOnly" wasn't deleted. How can I delete this field from every array and every document?

Answer 1

In the Mongo Query Language the easiest way you can express this update is using the $[] (Positional All) operator. It looks something like this:

db.col.update({},{$unset:{"someArray.$[].IsReadOnly":1}},{multi:true})

However, this operator has not been explicitly implemented in the strongly typed Builders available in the C# Driver.

Since this operator is missing you have to build the update "by hand". Here's a quick sample.

var mongo = new MongoClient();
var db = mongo.GetDatabase("test");
var col = db.GetCollection<BsonDocument>("col");
await col.UpdateManyAsync(new BsonDocument(), Builders<BsonDocument>.Update.Unset("someArray.$[].IsReadOnly"));

There are some potential pitfalls here with field naming if you have some custom serialization settings (such as lower camel cased field names), so be sure to test things.

It is worth noting that this answer is contingent upon you running a version of MongoDB that is 3.6 or newer. If you are running an older version, this method will not work as the $[] operator does not exist in earlier versions.

Answer 2

Hi Have you tried with MongoDb.Driver.Linq .. with something like:

var myEntity = myDb.Collection.AsQueryable().Where(xx=> xx.MyProp == somthing).FirstOrDefault(); //<-- to Extract your entity with Array Inside

then ..

var myArray = myEntity.ArrayNested.Where(xx=> xx.prop != conditiontoMatchElement).ToList();

then .. reset your new array (without the element) inside your entity..

myentity.ArrayNested = myArray;

.. then update it ...

var result = myDb.Collection.ReplaceOne(x => x.Id.Equals(myentity.Id), myentity, new UpdateOptions
            {
                IsUpsert = false
            });

Hope it helps you!!..

P.S . if you Array is not nested you can do the same but without set it in the father Entity

Answer 3

In MongoDB you can reach that with the following script:

db.getCollection('Collection').find({},{'someArray.IsReadOnly':0})

Key point is {'someArray.IsReadOnly':0} - projection parameter for find() method.

Read about find() here - https://docs.mongodb.com/manual/reference/method/db.collection.find/index.html

Answer 4

here's how to do it in a strongly typed manner using my library MongoDB.Entities. it enlists the help of ElemMatch and Unset definition builder methods.

using MongoDB.Entities;

namespace StackOverflow
{
    public class Program
    {
        public class User : Entity
        {
            public string Name { get; set; }
            public Post[] Posts { get; set; }
        }

        public class Post : Entity
        {
            public string Title { get; set; }
            public bool IsReadOnly { get; set; }
        }

        static void Main(string[] args)
        {
            new DB("test");

            var posts = new[] {
                new Post{ Title= "first post", IsReadOnly= true},
                new Post {Title = "second post", IsReadOnly = false}
            };
            posts.Save();

            (new User
            {
                Name = "Test User",
                Posts = posts
            }).Save();

            DB.Update<User>()
              .Match(f => f.ElemMatch(u => u.Posts, p => p.IsReadOnly == true || p.IsReadOnly == false))
              .Modify(d => d.Unset(u => u.Posts[-1].IsReadOnly))
              .Execute();
        }
    }
}

the following update command is sent to mongodb:

db.User.update(
    {
        "Posts": {
            "$elemMatch": {
                "$or": [
                    {
                        "IsReadOnly": true
                    },
                    {
                        "IsReadOnly": false
                    }
                ]
            }
        }
    },
    {
        "$unset": {
            "Posts.$.IsReadOnly": NumberInt("1")
        }
    }
)