MongoDB C＃驱动程序2.0更新集合并忽略重复项

Question

I am very new with MongoDB (only spend a day learning). I have a relatively simple problem to solve and I choose to take the opportunity and learn about this popular nosql database. In C# I have the following classes:

public class Item
{
    [BsonId]
    public string ItemId { get; set; }
    public string Name { get; set; }
    public ICollection<Detail> Details { get; set; }
}

public class Detail
{
    //[BsonId]
    public int DetailId { get; set; }
    public DateTime StartDate { get; set; }
    public double Qty { get; set; }
}

I want to be able to add multiple objects (Details) to the Details collection. However I know that some of the items I have (coming from a rest api) will already be stored in the database and I want to avoid the duplicates. So far I can think of 2 ways of doing it, but I am not really happy with either:

1. Get all stored details (per item) from MongoDB and then in .net I can filter and find the new items and add them to the db. This way I can be sure that there will be no duplicates. That is however far from ideal solution.

2. I can add [BsonId] attribute to the DetailId (without this attribute this solution does not work) and then use AddToSetEach. This works and my only problem with that is that I don't quite understand it. I mean, it suppose to only add the new objects if they do not already exists in the database, but how does it know? How does it compare the objects? Do I have any control over that comparison process? Can I supply custom comparers? Also I noticed that if I pass 2 objects with the same DetailId (this should never happen in the real app), it still adds both, so BsonId attribute does not guarantee uniqueness?

Is there any elegant solution for this problem? Basically I just want to update the Details collection by passing another collection (which I know that contain some objects already stored in the db ie first collection) and ignore all duplicates.

Answer 1

The AddToSetEach based version is certainly the way to go since it is the only one that scales properly.

I would, however, recommend you to drop the entire DetailId field unless it is really required for some other part of your application. Judging from a distance it would appear like any entry in your list of item details is uniquely identifiable by its StartDate field (plus potentially Qty , too). So why would you need the DetailId on top of that?

That leads directly to your question of why adding a [BsonId] attribute to the DetailId property does not result in guaranteed uniqueness inside your collection of Detail elements. One reason is that MongoDB simply cannot do it (see this link ). The second reason is that MongoDB C# driver does not create an unique index or attempts other magic in order to ensure uniqueness here - probably because of reason #1. ;) All the [BsonId] attribute does is tell the driver to serialize the attributed property as the "_id" field (and write the other way round upon deserialization).

On the topic of "how does MongoDB know which objects are already present", the documentation is pretty clear:

If the value is a document, MongoDB determines that the document is a duplicate if an existing document in the array matches the to-be-added document exactly; ie the existing document has the exact same fields and values and the fields are in the same order. As such, field order matters and you cannot specify that MongoDB compare only a subset of the fields in the document to determine whether the document is a duplicate of an existing array element.

And, no, there is no option to specify custom comparers.