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message_set of Django model

i'm new with Django and as I read the code, I don't understand the message_set attribute of Django model(called Room):
def room(request, pk):
room = Room.objects.get(id=pk)
**room_messages = room.message_set.all()**
participants = room.participants.all()
portion of Models:
class Room(models.Model):
host = models.ForeignKey(User, on_delete=models.SET_NULL, null=True)
topic = models.ForeignKey(Topic, on_delete=models.SET_NULL, null=True)
name = models.CharField(max_length=200)
description = models.TextField(null=True, blank=True)
participants = models.ManyToManyField(
User, related_name='participants', blank=True)
updated = models.DateTimeField(auto_now=True)
created = models.DateTimeField(auto_now_add=True)
class Message(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE)
room = models.ForeignKey(Room, on_delete=models.CASCADE)
body = models.TextField()
updated = models.DateTimeField(auto_now=True)
created = models.DateTimeField(auto_now_add=True)

If you define a ForeignKey from Message to Room, Django will add a relation in reverse to the from the Room model to its related Messages. By default this relation is named modelname_set with modelname the name of the origin of the model. You can specify another name by overriding the related_name=… parameter [Django-doc].
If you thus access the relation in reverse, you get all Message objects with room as there room, an equivalent query to room.message_set.all() is thus Message.objects.filter(room=room).

NameError: name 'Profile' is not defined

here is my models.py code. im trying to run the python3.8 manage.py migrate command to create the tables for the database but i keep getting this error, what could be the issue here. Profile is a class in the models.py code. if you need another part of my code please ask
from django.db import models
from django.contrib.auth.models import User
from django.utils import timezone
class Image(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE, null='True', blank=True)
image = models.ImageField(upload_to = 'pics/')
name = models.CharField(max_length=50,blank=True)
caption = models.CharField(max_length=250, blank=True)
likes = models.ManyToManyField(User, related_name='likes', blank=True, )
date_posted = models.DateTimeField(default=timezone.now)
class Comment(models.Model):
comment = models.TextField()
image = models.ForeignKey('Image', on_delete=models.CASCADE,related_name='comments',null='True', blank=True )
name = models.CharField(max_length=100, blank=True)
user = models.ForeignKey(Profile, on_delete=models.CASCADE, related_name='comments',null='True', blank=True )
created = models.DateTimeField(auto_now_add=True, null=True)
class Profile(models.Model):
name = models.OneToOneField(User, on_delete=models.CASCADE)
profile_picture = models.ImageField(upload_to='images/', default='default.png')
bio = models.TextField(max_length=500, default="My Bio", blank=True)
followers = models.ManyToManyField(User, related_name="followers", blank=True)
following = models.ManyToManyField(User, related_name="following", blank=True)

You are using the Profile class before defining it. Switch the order of the Comment class and Profile class. Like so:
class Profile(models.Model):
name = models.OneToOneField(User, on_delete=models.CASCADE)
profile_picture = models.ImageField(upload_to='images/', default='default.png')
bio = models.TextField(max_length=500, default="My Bio", blank=True)
followers = models.ManyToManyField(User, related_name="followers", blank=True)
following = models.ManyToManyField(User, related_name="following", blank=True)
class Comment(models.Model):
comment = models.TextField()
image = models.ForeignKey('Image', on_delete=models.CASCADE,related_name='comments',null='True', blank=True )
name = models.CharField(max_length=100, blank=True)
user = models.ForeignKey(Profile, on_delete=models.CASCADE, related_name='comments',null='True', blank=True )
created = models.DateTimeField(auto_now_add=True, null=True)

You are referencing the Profile class before this is constructed. You can make use of a string literal instead:
class Comment(models.Model):
# …
user = models.ForeignKey(
'Profile', # ← a string literal
on_delete=models.CASCADE,
related_name='comments',
null='True',
blank=True
)
# …
It might also be better to rename the field to profile, to make it clear the ForeignKey is referencing a Profile object, not a User object:
class Comment(models.Model):
# …
profile = models.ForeignKey( # ← rename to profile
'Profile',
on_delete=models.CASCADE,
related_name='comments',
null='True',
blank=True
)
# …
Note: It is normally better to make use of the settings.AUTH_USER_MODEL [Django-doc] to refer to the user model, than to use the User model [Django-doc] directly. For more information you can see the referencing the User model section of the documentation.

Django Rest Framework Integrity Error at NOT NULL constraint fail

i'm trying to post a transaction via django rest framework, however it shows error in django log as below:
IntegrityError at /api/item_trans/
NOT NULL constraint failed: chemstore_itemtransaction.bin_code_id
it has no problem if I post the same data from the Django admin web.
therefore I suppose the problem has happened at DRF
any help is welcome, thank you
models.py
class BinLocation(models.Model):
bin_code = models.CharField(max_length=10, unique=True)
desc = models.CharField(max_length=50)
def __str__(self):
return self.bin_code
class Meta:
indexes = [models.Index(fields=['bin_code'])]
class ItemMaster(models.Model):
item_code = models.CharField(max_length=20, unique=True)
desc = models.CharField(max_length=50)
long_desc = models.CharField(max_length=150, blank=True)
helper_qty = models.DecimalField(max_digits=10, decimal_places=4)
unit = models.CharField(max_length=10, blank=False)
def __str__(self):
return self.item_code
class Meta:
verbose_name = "Item"
verbose_name_plural = "Items"
indexes = [models.Index(fields=['item_code'])]
class ItemTransaction(models.Model):
# trace_code YYMMDDXXXX where XXXX is random generated
trace_code = models.CharField(max_length=20, unique=False)
item_code = models.ForeignKey(
ItemMaster, on_delete=models.CASCADE, related_name='+', blank=False, null=False)
datetime = models.DateTimeField(auto_now=False, auto_now_add=False)
qty = models.DecimalField(max_digits=10, decimal_places=4)
unit = models.CharField(max_length=10, blank=False)
action = models.CharField(
max_length=1, choices=ACTION, blank=False, null=False)
bin_code = models.ForeignKey(
BinLocation, related_name='+', on_delete=models.CASCADE, blank=False, null=False)
remarks = models.TextField(blank=True)
def __str__(self):
return f"{self.trace_code} {self.datetime} {self.item_code} {dict(ACTION)[self.action]} {self.qty} {self.unit} {self.bin_code}"
serializers.py
class ItemMasterSerializer(serializers.ModelSerializer):
class Meta:
model = ItemMaster
fields = '__all__'
class ItemTransactionSerializer(serializers.ModelSerializer):
item_code = serializers.SlugRelatedField(
slug_field='item_code',
read_only=True
)
bin_code = serializers.SlugRelatedField(
slug_field='bin_code',
read_only=True,
allow_null=False
)
class Meta:
model = ItemTransaction
fields = '__all__'

You might need to use 2 fields, one for reading data and the other for creating and updating your data with its source to the main. In your case you could try this:
class ItemTransactionSerializer(serializers.ModelSerializer):
item_code_id = ItemMasterSerializer(read_only=True)
item_code = serializers.PrimaryKeyRelatedField(
queryset=ItemMaster.objects.all(),
write_only=True,
source='item_code_id'
)
bin_code_id = BinLocationSerializer(read_only=True
bin_code = serializers.PrimaryKeyRelatedField(
queryset= BinLocation.objects.all(),
write_only=True,
source='bin_code_id'
)
Since you have null=False in both of your ForeignKeys, DRF expects the corresponding ID. You seem to be getting the error NOT NULL constraint because you are not passing the ID in DRF. So you need to fix that for both bin_code_id and the item_code_id.

AttributeError: Cannot use remove() on a ManyToManyField which specifies an intermediary model. Use songs.Playlist_Activity's Manager instead

I followed the documentation on figuring out how to remove an item in the manytomanyfield below but it doesn't seem to work and I am getting an attribute error. The remove method works totally fine without the intermediary model.
models.py
class Song (models.Model):
author=models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, default=1)
timestamp= models.DateTimeField(auto_now_add=True, editable=False)
songname = models.CharField(max_length=500, null=False, blank=False)
tags = TaggableManager()
def __str__(self):
return self.songname
class Playlist (models.Model):
name = models.CharField(max_length=150, null=False, blank=False)
author = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, default=1)
song = models.ManyToManyField(Song, through='Playlist_Activity')
class Playlist_Activity(models.Model):
song= models.ForeignKey(Song, on_delete=models.CASCADE)
playlist= models.ForeignKey(Playlist, on_delete=models.CASCADE)
timestamp= models.DateTimeField(auto_now_add=True, editable=False)
class Meta:
ordering = ('timestamp',)
Views.py
#login_required
def Playlist_Remove(request, id, P_id):
p = Playlist.objects.get(id=P_id)
s = p.song.get(id=id)
p.song.remove(s)
return redirect('account')

You can remove the intermediary instance:
p = Playlist.objects.get(id=P_id)
s = Song.objects.get(id=id)
Playlist_Activity.objects.get(song=s, playlist=p).delete()

Django: exclude objects based on another model

I have a model Comment:
class Comment(models.Model):
upload = models.ForeignKey(Upload, related_name='comments', on_delete=models.CASCADE)
user = models.ForeignKey(get_user_model(), related_name='comments', on_delete=models.CASCADE)
text = models.TextField(blank=False, null=False)
date_created = models.DateTimeField(auto_now_add=True)
And I also have a model BlockedUser:
class BlockedUser(models.Model):
blocked_by = models.ForeignKey(get_user_model(), on_delete=models.CASCADE, related_name="blocked_by")
user = models.ForeignKey(get_user_model(), on_delete=models.CASCADE)
date_created = models.DateTimeField(auto_now_add=True)
How can I fetch all the comments except ones written by someone who you've blocked?
queryset = Comment.objects.all().select_related('user')
queryset = queryset.exclude(user__in=BlockedUser.objects.filter(blocked_by=self.request.user))
Which obviously doesn't work like that, but I am not sure how to write it so that it does work.

You can exclude Comments from a user for which there exists a BlockedUser with blocked_by the request.user with:
Comment.objects.exclude(user__blockeduser__blocked_by=request.user)