Mongo C# 驱动程序 - 长度支持？

Question

The docs says I can use :

Find(x => x.FavoriteNumbers.Length == 3);

If I write:

await col.Find(f=>f.AuthType > 2).ToListAsync()

I do get the expected result because all AuthType are greater than 2 .

But now, let's say that I want to filter all records with Id.Length>2 :

await col.Find(f=>f.Id.Length>2).ToListAsync();

This returns 0 records.

Question:

How can I get all the records with Id's Length >2 ?

I've already read this question , but with no answer and the dup is not in c#

Answer 1

In Visual Studio you can write below line of code and try to inspect it by hovering:

var query = await col.Find(f=>f.Str.Length>2)

What happens is that MongoDB will transalte it into regular expression like {find({ "Str" : /^.{3,}$/s })} and this regex gets executed on the database side. Nothing unexpected. The problem occurs when you change Str into Id which gets translated into:

{find({ "_id" : /^.{3,}$/s })}

and you get no result as your field's name is still Id . The reason why it's happening is described here . So MongoDB driver by convention infers that field named Id should be translated into _id . To fix that you can explicitly specify on your data model which field should be considered as _id using BsonId attribute:

public class ModelClass
{
    [BsonId]
    public ObjectId RealId { get; set; }
    public string Id { get; set; }
}

EDIT:

To apply type conversion (int -> string) you have to rely on $toString operator introduced in MongoDB. The idea is simple: you add new field using $addFields and then run $strLenCP on it to verify string length. Unfortunately there's no easy way to do it in C# using strongly typed way so you can use BsonDocument class which lets you use strings as Aggregation Pipeline definition:

var q = Col.Aggregate()
            .AppendStage<BsonDocument>(BsonDocument.Parse("{ $addFields: { AuthTypeInt: { $toString: \"$AuthType\" } } }"))
            .Match(BsonDocument.Parse("{ $expr: { $gt: [ { \"$strLenCP\": \"$AuthTypeInt\" }, 2 ] } }"));

var data = await q.ToListAsync();

The code looks ugly but it should be a way faster than running plain JavaScript (more here )

Answer 2

I've managed to do it in a very ugly way, involving JS:

var ll=new BsonDocument("$where", new BsonJavaScript("function() { return this.authType.toString().length>2; }"));

var t2 = await col.Find(ll).ToListAsync();

What is the C# equivalent solution?