title: I am getting Integrity error in django rest framework
body: <p>I am begginer in django. I would like to add some posts and comments but I am getting an Integrity error.
  Without comments model it was working before but it doesn´t work together. I already delete my database and makemigrations and migrate again.</p>
  <p>post models</p>
  <pre><code>from django.db import models
  from django.conf import settings
  # from django.contrib.auth import get_user_model
  
  
  # User = get_user_model()
  
  # Create your models here.
  class Post(models.Model):
      user = models.ForeignKey(
          #to=User,
          to=settings.AUTH_USER_MODEL,
          on_delete=models.CASCADE,
          related_name='posts',
          null=True
      )
      content = models.CharField(
          max_length=150,
          blank=False
      )
      created = models.DateTimeField(
          auto_now=True
      )
      liked_by = models.ManyToManyField(
           #to=User,
           to=settings.AUTH_USER_MODEL,
           related_name='liked_posts',
           blank=True
      ) 
  </code></pre>
  <p>post serializer</p>
  <pre><code> from rest_framework import serializers
  from .models import Post
  from ..comment.serializers import CommentSerializer
  
  
  class PostSerializer(serializers.ModelSerializer):
      class Meta:
          model = Post
          comments = CommentSerializer(source='comments.content')
          fields = [
              'id',
              'user',
              'content',
              'comments',
              'created',
              'liked_by',
          ]
  </code></pre>
  <p>comment.models</p>
  <pre><code>from django.db import models
  from django.conf import settings
  from apps.post.models import Post
  
  
  # Create your models here.
  class Comment(models.Model):
      user = models.ForeignKey(to=settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name='comment', null=True)
      post = models.ForeignKey(to=Post, on_delete=models.SET_NULL, related_name='comment', null=True)
      content = models.CharField(max_length=150)
      created = models.DateTimeField(auto_now_add=True)
  
      def __str__(self):
          return f'Comment by: {self.user}'
  </code></pre>
  <p>comment serializer</p>
  <pre><code>from rest_framework import serializers
  from .models import Comment
  
  
  class CommentSerializer(serializers.ModelSerializer):
      class Meta:
          model = Comment
          fields = ['id', 'user', 'post', 'content', 'created']
  </code></pre>