Mutation in django graphene for model with foreign key and many to many relationship

I have 2 models in my django app, the first is Tags model and the second is Post model, the problem is when i try to use mutation for the Post model to add a post from the graphql it doesn't work but it works fine in the Tags model, also the Queries works fine when i try to get data from the database.

Here's my Code: model.py:

from django.db import models
from django.utils.translation import gettext as _
from django.conf import settings
from django.contrib.auth import get_user_model

User = get_user_model()
# Create your models here.

class Tag(models.Model):
  name    = models.CharField(_("Tag Name"), max_length=50, unique=True)
  
  def __str__(self):
    return self.name  


class Post(models.Model):
  title           = models.CharField(_("Title"), max_length=50, unique=True)
  slug            = models.SlugField(_("Post Slug"), unique=True)
  body            = models.TextField(_("Post Content"))
  createdAt       = models.DateTimeField(auto_now_add=True)
  updatedAt       = models.DateTimeField(auto_now=True)
  published       = models.BooleanField(_("Published"), default=False)
  author          = models.ForeignKey(User, verbose_name=_("Author"), on_delete=models.CASCADE)
  tags            = models.ManyToManyField("Tag", verbose_name=_("Tags"), blank=True)

  class Meta:
    ordering = ['-createdAt']
  
  def __str__(self):
    return self.title

schema.py:

import graphene
from graphene_django import DjangoObjectType
from django.contrib.auth import get_user_model
from .models import Post, Tag

User = get_user_model()

class PostType(DjangoObjectType):
  class Meta:
   model = Post

class TagType(DjangoObjectType):
  class Meta:
   model = Tag
   fields = ('id', 'name',)

class TagInput(graphene.InputObjectType):
   name = graphene.String(required=True)
   
class CreatePostMutation(graphene.Mutation):
   class Arguments:
      title    = graphene.String()
      body     = graphene.String()
      published  = graphene.Boolean()
      author_id  = graphene.ID()
      tags     = graphene.List(graphene.Int) 
   
   success = graphene.Boolean()
   post = graphene.Field(PostType)
   
   @classmethod
   def mutate(cls, root, info, title, body, published, author_id, tags):
      author = User.objects.get(pk=author_id)
      post_instance = Post(
         title=title,
         body=body,
         published=published,
         author=author,
      )
      post_instance.save(commit=False)

      if tags:
         tag_objects = Tag.objects.filter(pk__in=tags)
         post_instance.tags.set(tag_objects)
      post_instance.save()

      return CreatePostMutation(success=True, post=post_instance)

class CreateTagMutation(graphene.Mutation):
   class Arguments:
      name = graphene.String(required=True)

   tag = graphene.Field(TagType)

   @classmethod
   def mutate(cls, root, info, name):
      tag = Tag(name=name)
      tag.save()
      return CreateTagMutation(tag=tag)

class Mutation(graphene.ObjectType):

   create_post = CreatePostMutation.Field()
   create_tag  = CreateTagMutation.Field()

schema = graphene.Schema(query=Query, mutation=Mutation)

Here's what it returns:

{
  "data": {
    "createPost": {
      "post": null
    }
  }
}

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