Question

I’m having trouble finding the right syntax to replace an array of objects nested several levels deep within a collection. My preference is to just update the individual properties, but since reading the link below, it seems replacing the whole array is the best bet.

https://jira.mongodb.org/browse/SERVER-831

So I have the following classes as an example:

public class Parent
    {
        public ObjectId _id { get; set; }
        public string Firstname { get; set; }
        public string Lastname { get; set; }
        public Collection<Child> Children { get; set; }
    }

    public class Child
    {
        public ObjectId _id { get; set; }
        public string Firstname { get; set; }
        public string Lastname { get; set; }
        public Collection<Pet> Pets { get; set; }
    }

    public class Pet
    {
        public string Name { get; set; }
    }

Using the following code I create a Parent, add some Children, and add a Pet to one of the children.

// Construct Objects
Parent parent = new Parent() { _id = new ObjectId("4f979621682dbc1a8cefecb1") };

Collection<Child> children = new Collection<Child>();
Collection<Pet> pets = new Collection<Pet>();

children.Add(new Child() 
    { _id = new ObjectId("4f979621682dbc1a8cefecaf"), 
      Firstname = "Child", 
      Lastname = "One" });
children.Add(new Child() 
    { _id = new ObjectId("4f979621682dbc1a8cefecb0"), 
      Firstname = "Child", 
      Lastname = "Two" });
pets.Add(new Pet() { Name = "Fishy" });

parent.Children = children;
parent.Children[0].Pets = pets;

// Connect to Mongo
var server = MongoServer.Create("mongodb://localhost/?safe=true");
var db = server.GetDatabase("test");

// Insert into parent collection
MongoCollection<Parent> parents;
parents = db.GetCollection<Parent>("parents");
parents.Insert<Parent>(parent, MongoDB.Driver.SafeMode.True);

This successfully inserts the objects, generating the following JSON result:

{   "_id" : ObjectId("4f979621682dbc1a8cefecb1"),
    "Firstname" : null,
    "Lastname" : null,
    "Children" : 
    [
        {
            "_id" : ObjectId("4f979621682dbc1a8cefecaf"),
            "Firstname" : "Child",
            "Lastname" : "One",
            "Pets" : 
            [
                {
                    "Name" : "Fishy"
                }
            ]
        },
        {
            "_id" : ObjectId("4f979621682dbc1a8cefecb0"),
            "Firstname" : "Child",
            "Lastname" : "Two",
            "Pets" : null
        }
    ]
}

Updating individual document elements also seems to be a trivial process, and works successfully with the following code.

// Change children's name
var query = new QueryDocument { { "Children._id", new ObjectId("4f979621682dbc1a8cefecaf") } };
var update = Update.Set("Children.$.Firstname", "Something");
parents.Update(query, update);

Now the issue I can’t work out is how to replace the Pets array. The following code doesn’t compile as Update.Set doesn’t accept a Collection.

// Change pets information
pets[0].Name = "Fishy2"; // change to pet
pets.Add(new Pet() { Name = "Doggy" }); // add new pet

query = new QueryDocument { { "Children._id", new ObjectId("4f979621682dbc1a8cefecaf") } };
update = Update.Set("Children.$.Pets", pets);
parents.Update(query, update);

So what’s the best process that would enable me to update the details in the Pets array?

Answer 1

Here is the code you are looking for: You’ll need to pass in a BsonArray to the Update.Set value. To create that array, you’ll need to wrap each of the “pets” in a BsonDocumentWrapper so the serialization library knows how to serialize them appropriately.

var query = new QueryDocument { { "Children._id", new ObjectId("4f979621682dbc1a8cefecaf") } };
var petDocuments = BsonDocumentWrapper.CreateMultiple(pets);
var petArray = new BsonArray(petDocuments);
var update = Update.Set("Children.$.Pets", petArray);
parents.Update(query, update);