Null value in column violates not-null constraint after deleting objects incorrectly

Question

Help me please I don't know what's going on. I wrote some simple blog, where I could add posts and comments. It was working well.

views.py:

def add_comment_to_post(request, pk):
    post = get_object_or_404(Post, pk=pk) # calls the given model and get the object, type = <class 'blog.models.Post'
    if request.method == "POST": # if we posted data
        form = CommentForm(request.POST) 
        if form.is_valid(): # if all fields are filled
            comment = form.save(commit=False) # create instance and return which not saved in database "Comment" <class Comment>
            comment.post = post # return Title
            comment.author = request.user
            comment.save()
            return redirect('post_detail', pk=post.pk)
    else:
        form = CommentForm() # type = <class 'blog.forms.CommentForm'>
    return render(request, 'blog/add_comment_to_post.html', {'form': form})

But when I added a function for deleting comments, that wrote me a mistake, like this:

null value in column "approved_comment" of relation "blog_comment" violates not-null constraint

views.py:

def comment_remove(request, pk):
       post = get_object_or_404(Post, pk=pk)
       comment = get_object_or_404(Comment, pk=pk)
       print(comment)
       comment.delete()
       return redirect('post_detail', pk=post.pk)

I think I deleted comment incorrectly. I don't understand how primary keys works, how comments and posts are related each other. And I don't understand how to understand it.

ps. models.py:

class Post(models.Model):
    author = models.ForeignKey(settings.AUTH_USER_MODEL, 
related_name="author_name",
on_delete=models.CASCADE)
    title = models.CharField(max_length=200)
    text = models.TextField()
    created_date = models.DateTimeField(default=timezone.now)
    published_date = models.DateTimeField(blank=True, null=True)

def publish(self):
    self.published_date = timezone.now()
    self.save()

def __str__(self):
    return self.title

def approved_comments(self):
    return self.comments.filter(approved_comment=True)

class Meta:
    ordering = ['-published_date']

class Comment(models.Model):
    post = models.ForeignKey('blog.Post', on_delete=models.CASCADE, 
related_name='comments')
    author = models.CharField(max_length=200)
    text = models.TextField()
    created_date = models.DateTimeField(default=timezone.now)
    approved_comment = models.BooleanField(default=False),
    # parent = models.ForeignKey('self', null=True, blank=True, 
related_name='replies')

def approve(self):
    self.approved_comment = True
    self.save()

def __str__(self):
    return 'Comment {} by {}'.format(self.text, self.author)

pps. forms.py:

from xml.etree.ElementTree import Comment
from django import forms
from .models import Post, Comment

class PostForm(forms.ModelForm):

    class Meta:
        model = Post
        fields = ('title', 'text',)

class CommentForm(forms.ModelForm):

    class Meta:
        model = Comment
        fields = ('text',)

Answer 1

To understand how to delete comments we need to understand how pk(primary keys) and related_name works.

Let's see model class Comment. Pay attention for post and related_name='comments' and than see it in post_detail.html.

class Comment(models.Model):
    post = models.ForeignKey('blog.Post', on_delete=models.CASCADE, 
related_name='comments')
    author = models.CharField(max_length=200)
    text = models.TextField()
    created_date = models.DateTimeField(default=timezone.now)
    approved_comment = models.BooleanField(default=False)

post_detail.html:

<a class="btn btn-default" href="{% url 'add_comment_to_post' pk=post.pk %}">Add comment</a>
    {% for comment in post.comments.all %}
        <div class="comment">
            <a class="btn btn-default" href="{% url 'comment_remove' pk=comment.pk %}"><span class="glyphicon glyphicon-remove"></span></a>
            <div class="date">{{ comment.created_date }}</div>
            <strong>{{ comment.author }}</strong>
            <p>{{ comment.text|linebreaks }}</p>
        </div>
    {% empty %}
        <p>No comments here yet :(</p>
    {% endfor %}

post.comments.all - post is relating for model our Post model. The related_name specifies the name of the relation in reverse, so in this case accessing the Comment of a given Post object. [https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.ForeignKey.related_name] Now you know how we get comment.pk.

After that please pay attention how we're removing comment. views.py:

def comment_remove(request, pk):
    comment = get_object_or_404(Comment, pk=pk)
    comment.delete()
    return redirect('post_detail', pk=comment.post.pk)

Look at the comment.post.pk. In our Comment model we have key post = models.ForeignKey('blog.Post', on_delete=models.CASCADE, related_name='comments'), which is relating with our Post Model. That's it!

Answer 2

It's better to send your code here. But below ways may help you.

How to Pass Data Between a Parent Component and a Child Component Firstly, let's pass data between a parent component and a child component.

First, you'll need to create two components, one parent and one child.

    import React from 'react'

    export default function Parent() {
    return (
    <div>
      
    </div>
  )
}

Parent.js

 import React from 'react'

     export default function Child() {
    return (
        <div>
            
        </div>
     )
    }

Next, you'll import the child component in the parent component and return it.

    import React from 'react'
    import Child from './Child';

    export default function Parent() {
    return (
    <div>
      <Child/>
    </div>
   )
   }

Then you'll create a function and a button to trigger that function. Also, you'll create a state using the useState Hook to manage the data.

import React from 'react'
import Child from './Child';
import { Button } from 'semantic-ui-react';
import { useState } from 'react';
import './App.css';

export default function Parent() {
  const [data, setData] = useState('');
  
  const parentToChild = () => {
    setData("This is data from Parent Component to the Child Component.");
  }
  return (
    <div className="App">
      <Child/>
      
      <div>
        <Button primary onClick={() => parentToChild()}>Click Parent</Button>
      </div>
    </div>
  )
}

read more

Answer 3

You could just read it from another component using AsyncStorage. There is no need for passing here.