Can I push to a nested array without knowing whether that array already exists?

Question

I have a data model that's structured something like this:

{
    "_id": "1234abcd",
    "name": "The Stanley Parable"
    "notes": [
        {
            "_id": "5678efgh",
            "content": "Kind of a walking simulator."
        },
                    {
            "_id": "5678efgh",
            "content": "Super trippy."
        },
    ]
}

I'm using the mongo driver for C# to interact with this model in .NET Core. I'm trying to add a new note to a particular game -- without necessarily being sure that there are any notes on that game already. This is what I've been able to find for adding to a nested list:

var filter = Builders<Mongo_VideoGame>.Filter.Eq("Id", id);
var update = Builders<Mongo_VideoGame>.Update.Push<Mongo_Note>(v => v.Notes, mongoNote);

var editedGame = _context.VideoGames.FindOneAndUpdate(filter, update);

But my problem is that this only works if there's already a "notes" element on the game model. I could just add an empty array when I make the original game, but that seems less flexible to changing the schema (i.e. what if I was adding notes to existing data).

An exception of type 'MongoDB.Driver.MongoCommandException' occurred in MongoDB.Driver.Core.dll but was not handled in user code: 'Command findAndModify failed: The field 'notes' must be an array but is of type null in document'

Tracy Moody · Accepted Answer · 2017-10-19 16:58:46Z

1

So I ended up finding a solution to this.

First off, .Push() does work as I originally expected. It will create an array if one doesn't already exist.

My actual problem had to do with the C# Model I was using. Previously:

public class Mongo_VideoGame
{
    [BsonId]
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }

    [BsonElement("name")]
    public string Name { get; set; }

    [BsonElement("notes")]
    public List<Mongo_Note> Notes { get; set; }

}

But since there was no notes on the document it found, I was basically trying to insert into a null notes array. So to fix this, I set the default value for Notes so that it could be pushed into. Like this:

public class Mongo_VideoGame
{
    [BsonId]
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }

    [BsonElement("name")]
    public string Name { get; set; }

    [BsonElement("notes")]
    public List<Mongo_Note> Notes { get; set; } = new List<Mongo_Note>();

}

Both the case of adding to an existing array and creating the array if it does not exist work now.

edited Oct 19, 2017 at 16:58

answered Oct 10, 2017 at 2:03

Tracy Moody

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1 Comment

Mike Beeler Over a year ago

defining the storage does not allocate the storage as you discovered. when using a DI container the hardwired new list would need to be moved. Perhaps to the constructor as constructor based injection is fairly common.

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