What does a function in a django model do?-CodePudding

i wrote this function in django model that would help send and get messages but tbh i do not understand what the code exactly does because the tutorial teacher didn't explain anything about the code and for days now i have been trying to figure out what the code does with no solutions yet. I do not just want to copy and paste code from a tutorial, i want to also understand what each code does so that's why i would appreciate anyhelp.

The code

def get_messages(user):
        messages = Message.objects.filter(user=user).values('recipient').annotate(last=Max('date')).order_by('-last')
        users = []
        for message in messages:
            users.append({
                'user': User.objects.get(pk=message['recipient']),
                'last': message['last'],
                'unread': Message.objects.filter(user=user, recipient__pk=message['recipient'], is_read=False).count()
                })
        return users

complete models.py

class Message(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE, related_name='user')
    sender = models.ForeignKey(User, on_delete=models.CASCADE, related_name='from_user')
    recipient = models.ForeignKey(User, on_delete=models.CASCADE, related_name='to_user')
    body = models.TextField(max_length=1000, blank=True, null=True)
    date = models.DateTimeField(auto_now_add=True)
    is_read = models.BooleanField(default=False)

    def send_message(from_user, to_user, body):
        sender_message = Message(
            user=from_user,
            sender=from_user,
            recipient=to_user,
            body=body,
            is_read=True)
        sender_message.save()

        recipient_message = Message(
            user=to_user,
            sender=from_user,
            body=body,
            recipient=from_user,)
        recipient_message.save()
        return sender_message

    def get_messages(user):
        messages = Message.objects.filter(user=user).values('recipient').annotate(last=Max('date')).order_by('-last')
        users = []
        for message in messages:
            users.append({
                'user': User.objects.get(pk=message['recipient']),
                'last': message['last'],
                'unread': Message.objects.filter(user=user, recipient__pk=message['recipient'], is_read=False).count()
                })
        return users

views.py if needed

def Inbox(request):
    messages = Message.get_messages(user=request.user)
    active_direct = None
    directs = None

    if messages:
        message = messages[0]
        active_direct = message['user'].username
        directs = Message.objects.filter(user=request.user, recipient=message['user'])
        directs.update(is_read=True)
        for message in messages:
            if message['user'].username == active_direct:
                message['unread'] = 0

CodePudding user response：

Message.objects.filter(user=user).values('recipient').annotate(last=Max('date')).order_by('-last')

This will select all Messages for a specific user, of those Message object you are selecting the recipient value and the max date. This is equal to the following in SQL:

SELECT
    "messages"."recipient",
    MAX("messages"."date") AS "last"
FROM
    "messages"
WHERE
    "messages"."recipient" = x
GROUP BY
    "messages"."recipient",
    "messages"."date"
ORDER BY
    "messages"."date" DESC

Then in the loop the user objects is retrieved by the recipient value:

'user': User.objects.get(pk=message['recipient']),

Unread will be set to the amount of messages that this user has sent to the given recipient but are still unread.

'unread': Message.objects.filter(user=user, recipient__pk=message['recipient'], is_read=False).count()

Tip: if you ever have a django query, you can just call see the SQL query by doing the following: print(Message.objects.filter(...).query)