从带有 Linq 表达式的 IMongoQueryable 集合中选择会引发错误 [c# 驱动程序 2.2.4]

Question

Using the mongoDb c# driver 2.2.4, selecting from a IMongoQueryable collection with a Linq Expression throws an error because of my Id field. Here is the code:

MongoDB.Driver.MongoClient myClient = new MongoDB.Driver.MongoClient("mongodb://localhost:27010");
            var db = myClient.GetDatabase("YOUR_MONGODB_DATABASE");
            var collection = db.GetCollection<Man>("Test");
            var queryableCollection = collection.AsQueryable();

            Man obj = new Man() {Id = "Man1_671", Name = "Patrick"};
            collection.InsertOneAsync(obj).Wait();
            // Insertion works fine

            System.Linq.Expressions.Expression<Func<Man, Man>> func = mec => new Man
            {
                Id = mec.Id,
                Name = mec.Name
            };

            // This line bugs using the 2.2.4 driver and works using the 2.2.1 version
            var listProjection = queryableCollection.Select(func).ToListAsync().Result;

Obviously I would have to rename my property to '_id' (tested, it works), but I don't get why it used to work (especially because it is stated that no breaking changes have been introduced). Was I using a bug that had been fixed ? Is it a bug in the driver ? Any help would be greatly appreciated :)

Edit 1: exception thrown is : Element 'Id' does not match any field or property of class MongoDbHierarchicalStoring.Man.

Regards, Florent

Answer 1

I've been able to work around this issue by using a class on top of the real type for the projection purposes:

internal class ProductProjection : Product
{ 
    [BsonIgnore]
    [JsonIgnore]
    public string _id { set => Id = value; }
}

It's then possible to project the aggregate query into this new type and implicitly cast it back:

List<Product> results = _collection.Aggregate().Match(queryFilter).Project(
    x => new ProductProjection
    {
        _id = x.Id,
        ....
    }).ToList();

The resulting Products contain any custom projections as well as the fully populated Id field.