Mongodb c# driver FindAll with setFields and AsQueryable

Question

Using MongoDB C# driver, seems that I'm unable to get the data by AsQueryable with setFields and Where condition only by mongo query. I fetched documents by this code

var query = _collection.FindAll().SetFields(fields.MongoFieldsBuilder).AsQueryable();
var query1 = query.Where(d=>d.Name="Ken").ToList();
var query2 = query.Where(d=>d.Age>=2).ToList();

So, when query1 or query2 executed, c# driver fetches all documents from mongo and then filters it in memory. But I expected that Where condition will be converted to mongo query with fields. Can anyone please explain how to do it correctly?

Answer 1

You using .AsQueryable() from System.Linq , because of this it filter everything on a client side. But you need to use .AsQueryable() from MongoCollection to filter data in a database. This extension method creates MongoQueryable<T> .

I believe that following should work:

//or you could use your projection class instead of BsonDocument
var query = Items.AsQueryable<BsonDocument>()   
                 .Select(x=> new {id = x["_id"].AsObjectId, Name=x["Name"].AsString});
var query1 = query.Where(d=>d.Name == "Ken").ToList();

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Update:

If you want use SetFields you have to use old query syntax:

_col.Find(Query<UserDocument>.EQ(x=> x.Name, "Ken")).SetFields(..). 

Also you could use SetFields without magic strings as follow:

cursor.SetFields(Fields<UserDocument>.Include(x=> x.Name, x=> x.Age))

With linq SetFields is done via Select .

Hope this helps.