[Answered ]-How to check if an object is in another queryset

Please don’t. While this can be done with such condition, it would result in a lot of extra queries. You can work with a Exists subquery ^[Django-doc]:

from django.contrib.auth.decorators import login_required
from django.db.models import Exists, OuterRef
from django.shortcuts import redirect

@login_required
def index(request):
    if request.method == 'POST':
        text = request.POST['post-text']
        Post.objects.create(
            entity = Entity.objects.create(
                user = request.user,
                text = text
            )
        )
        return redirect('index')
    posts = Post.objects.annotate(
        is_liked=Exists(
            Like.objects.filter(user=request.user, entity_id=OuterRef('entity_id')
        )
    ).select_related('entity').order_by('-entity__date', '-entity__id')
    
    return render(request, "network/index.html", {
        'posts' : posts
    })

The Post objects will then have an extra attribute .is_liked that is True if that Post is liked by the logged in user.

Note: You can limit views to a view to authenticated users with the
@login_required decorator ^[Django-doc].

Note: You can make use of redirect(…) ^[Django-doc] instead
of first calling reverse(…) ^[Django] and
then wrap it in a HttpResponseRedirect object ^[Django-doc].
The redirect(…) function does not only offer a more convenient signature to do this, it also for example will use the
.get_absolute_url() method ^[Django-doc]
if you pass it a model object.

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.

Note: Django’s DateTimeField ^[Django-doc]
has a auto_now_add=… parameter ^[Django-doc]
to work with timestamps. This will automatically assign the current datetime
when creating the object, and mark it as non-editable (editable=False), such
that it does not appear in ModelForms by default.