[Fixed]-How to add extra fields to related model before Saving to database?

Thanks @dan-klasson for the Help.

I finally figured Out How to do this :

Firstly in My CommentsInline Class i had to add a method to update the formsets like this :

admin.py

from django.core import serializers
from django.http import HttpResponse
from django.utils import timezone
from django.contrib import admin
from .models import Post, Comment
from .forms import CommentInlineFormset


# Register your models here.

class CommentsInline(admin.StackedInline):
    model = Comment
    extra = 1
    fields = ['comment_text']
    formsets = CommentInlineFormset

    def get_formset(self, request, obj=None, **kwargs):
        formset = super(YourInline, self).get_formset(request, obj, **kwargs)
        formset.request = request
        return formset



class PostAdmin(admin.ModelAdmin):
    fieldsets = [
        ('Content', {'fields': ('title', 'body', 'is_published')}),
        ('Date Information', {'fields': ('pub_date', )})
    ]

    inlines = [CommentsInline]

    def save_model(self, request, obj, form, change):
        obj.author = request.user
        if not obj.id:
            obj.created_date = timezone.now()
        obj.updated_date = timezone.now()
        super(PostAdmin, self).save_model(request, obj, form, change)


admin.site.register(Post, PostAdmin)

and In my forms.py

from django import forms
from django.utils import timezone


class CommentInlineFormset(forms.models.BaseInlineFormSet):
    def save_new(self, form, commit=True):
        obj = super(CommentInlineFormset, self).save_new(form, commit=False)
        # here you can add anything you need from the request
        obj.user = self.request.user
        obj.created_on = timezone.now()

        if commit:
            obj.save()

        return obj