How to Update Comment List in Jinja2 Templates Using WebSockets with FastAPI?

How to Get the Updated List of Items in Jinja2 Template Using FastAPI?

Problem:

Consider a scenario where you're building a commenting system on a blog. You're rendering existing comments using Jinja2 templates. When a new comment is posted via a FastAPI route, you need to update the list of comments displayed in the template.

Solution:

While Jinja2 is not ideal for handling real-time updates, a solution can be found using WebSockets. This requires modifying both the FastAPI backend and the Jinja2 template:

FastAPI Backend (app.py):

• Create a ConnectionManager class to manage WebSocket connections.
• Define a send_personal_message method to send messages to specific clients.
• Define a broadcast method to send messages to all connected clients.
• Update the / route to handle connections and pass the comments list to the template.
• Create a WebSocket endpoint (/ws) to handle incoming WebSocket connections and broadcast new comments.

Jinja2 Template (index.html):

• Add a
  element to display the comments.
  • Create a JavaScript function addComment to send new comments via WebSocket.
  • In the onmessage event handler for the WebSocket, create HTML elements to display the new comment and append it to the
    element.

    Example Code:

    <code class="html"><!-- app.py -->
    @app.websocket("/ws")
    async def websocket_endpoint(websocket: WebSocket):
        await manager.connect(websocket)
        try:
            while True:
                data = await websocket.receive_json()
                comments.append(Comment(data['author'], data['content']))
                await manager.broadcast(data)
        except (WebSocketDisconnect, ConnectionClosed):
            manager.disconnect(websocket)</code>

    <code class="html"><!-- index.html -->
    <script>
        var ws = new WebSocket("ws://localhost:8000/ws");
        ws.onmessage = function(event) {
            var comments = document.getElementById('comments')
            var comment = document.createElement('li')
            var jsonObj  = JSON.parse(event.data);
            var authorNode = document.createElement('h3');
            authorNode.innerHTML = jsonObj.author;
            var contentNode = document.createElement('p');
            contentNode.innerHTML = jsonObj.content;
            comment.appendChild(authorNode);
            comment.appendChild(contentNode);
            comments.appendChild(comment)
        };
        function addComment(event) {
            var author = document.getElementById("author")
            var content = document.getElementById("content")
            ws.send(JSON.stringify({
                "author": author.value,
                "content": content.value
            }))
            author.value = ''
            content.value = ''
            event.preventDefault()
        }
    </script></code>

    By utilizing WebSockets, you can establish a real-time connection between the FastAPI backend and the Jinja2 template, allowing for the updated list of comments to be displayed in the UI.

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