Question

Here is the situation. I am inserting a new post and after insert I fetch the post and it works fine. Then I change one field and update which works fine. The problem occurs when I try to fetch the same post after the update. It always returns null.

     public class Post
        {
            public string _id { get; set; }
            public string Title { get; set; }
            public string Body { get; set; }
        }

 // insert a post 
        var post = new Post() {Title = "first post", Body = "my first post"};
        var posts = _db.GetCollection("posts");
        var document = post.ToDocument();

        // inserts successfully! 
        posts.Insert(document);

        // now get the post 
        var spec = new Document() {{"_id", document["_id"]}};

        // post was found success
        var persistedPost = posts.FindOne(spec).ToClass<Post>();

        persistedPost.Body = "this post has been edited again!!";
        var document2 = persistedPost.ToDocument(); 
        // updates the record success although I don't want to pass the second parameter
        posts.Update(document2,spec);

        // displays that the post has been updated
        foreach(var d in posts.FindAll().Documents)
        {
            Console.WriteLine(d["_id"]); 
            Console.WriteLine(d["Body"]);
        }

    // FAIL TO GET THE UPDATED POST. THIS ALWAYS RETURNS NULL ON FindOne call! 
    var updatedPost = posts.FindOne(new Document() {{"_id",document["_id"]}}).ToClass<Post>(); // this pulls back the old record with Body = my first post
    Assert.AreEqual(updatedPost.Body,persistedPost.Body);

UPDATE:

I think I have resolved the problem but the issue is very weird. See the last line.

var updatedPost = posts.FindOne(new Document() {{"_id",document["_id"]}}).ToClass<Post>();

The FindOne method takes new document which depends on document[“_id”]. Unfortunately, that does not work and for some reason it requires you to send the _id associated with persistedPost update which you will get after the update command. Here is the example:

 var persistedPost = posts.FindOne(spec).ToClass<Post>();
            persistedPost.Body = "this is edited";
            var document2 = persistedPost.ToDocument(); 
            posts.Update(document2,new Document() {{"_id",document["_id"]}});

            var updatedPost = posts.FindOne(new Document(){{"_id",document2["_id"]}}).ToClass<Post>(); 
            Console.WriteLine(updatedPost.Body);

See, now I am sending the document2[“_id”] instead of the document field. This seems to work correctly. I guess the 24 byte code that it generates for each “_id” field is different.

Answer 1

The answer is that not to rely on the “_id” generated by MongoDb. Just use your own unique identifier like Guid or identity.

UPDATE:

My ToDocument method was putting the _id as string you must always put _id as Oid.