Sure! Here's a possible schema for the "messages" collection in the MongoDB database:
```
{
  "src_username": <string>,
  "dst_username": <string>,
  "date_time": <datetime>,
  "message": <string>
}
```

The "src_username" field represents the username of the message sender, while the "dst_username" field represents the username of the message recipient. The "date_time" field represents the date and time when the message was sent, and the "message" field represents the content of the message.

You can use this schema to create the "messages" collection in MongoDB and insert new messages into it using the PyMongo driver in your Python backend. For example, you can use the following code to insert a new message:

```
from pymongo import MongoClient
import datetime

# Connect to the MongoDB database
client = MongoClient("<your MongoDB URI>")
db = client["<your database name>"]
messages = db["messages"]

# Insert a new message
message = {
  "src_username": "user1",
  "dst_username": "user2",
  "date_time": datetime.datetime.now(),
  "message": "Hello, how are you?"
}
result = messages.insert_one(message)
```

This code connects to the MongoDB database using the PyMongo driver, creates a new message object with the sender, recipient, date/time, and message content, and inserts it into the "messages" collection using the insert_one() method. You can use similar code to retrieve chat history from the "messages" collection and display it on the chat interface.
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