I was trying to implement MongoDB in my Unity WebGL game and everything is working fine in the editor: I managed to insert and retrieve documents from my Mongo database with no issues.

Everything seemed perfect until I compiled the project and realized that my WebGL build won't communicate with my database at all!

I took a look at the console and I see this error:

MissingMethodException: Default constructor not found for type MongoDB.Bson.Serialization.BsonClassMap`1[[UserAccount, Assembly-CSharp, Version=, Culture=neutral, PublicKeyToken=null]]

I have no clue what's happening. I tried compiling the project for Windows/Linux and I got the same error, then I changed API compatibility level to 4.x and it was working fine on the Windows build, but not on WebGL ( API compatibility level=4.x is checked for WebGL build!)

Can someone lend me a hand with that?

  • \$\begingroup\$ You need your app to connect to websocket/http server(Preferably node.js) which will then connect to mongodb. This will also add security to the app. \$\endgroup\$
    – Nick
    Commented May 6, 2021 at 16:17

1 Answer 1


What you are trying to do here unfortunately can not work. The MongoDB protocol is based directly on TCP/IP. But a web application running in a web browser can not create raw TCP/IP connections. There is no API in JavaScript for doing that. The closest it gets are HTTP requests ("AJAX") and WebSockets (which are intentionally incompatible with regular sockets). A WebGL build is a JavaScript application, so if the JavaScript API can't do something, then Unity won't magically allow you to do it.

If you want to use a MongoDB database from your Unity application running in the browser, then you will have to use a middleware which provides a REST API for MongoDB which you can then access from your game by using the UnityWebRequest class.

  • \$\begingroup\$ that explains everything, it seems I have plenty of work ahead :(, thank you a lot for your help! you saved me! \$\endgroup\$ Commented Apr 10, 2019 at 13:19
  • \$\begingroup\$ @roseicollis Are you already committed to using MongoDB as your database? If you are looking for a document-oriented NoSQL database which offers a REST API out-of-the-box, then you might want to take a look at CouchDB. \$\endgroup\$
    – Philipp
    Commented Apr 10, 2019 at 13:29
  • \$\begingroup\$ I'll take a look at that, thanks for the suggestion! \$\endgroup\$ Commented Apr 10, 2019 at 14:06
  • \$\begingroup\$ @roseicollis When this post answered your question, please accept it by clicking the checkmark-icon next to it. \$\endgroup\$
    – Philipp
    Commented Apr 11, 2019 at 8:06
  • \$\begingroup\$ For future reference, I faced the same problems myself, and implemented what Philipp suggested quite recently. I have written a short github repo that details how to connect Unity to a MongoDB Atlas VM here \$\endgroup\$ Commented May 6, 2021 at 15:47

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