Django template: check for empty query set

Question

Is there a way to check for an empty query set in the Django template? In the example below, I only want the NOTES header to be displayed if there are notes.

If I put an {% empty %} inside the "for" then it does display whatever is inside the empty tag, so it knows it's empty.

I'm hoping for something that does not involve running the query twice.

{% if notes - want something here that works %}
     NOTES:
     {% for note in notes %}
         {{note.text}}  
     {% endfor  %}
{% endif  %}

Clarification: the above example "if notes" does not work - it still displays the header even with an empty query set.

Here's a simplified version of the view

sql = "select * from app_notes, app_trips where"
notes = trip_notes.objects.raw(sql,(user_id,))

return render_to_response(template, {"notes":notes},context_instance=RequestContext(request))  

Edit: the view select selects from multiple tables.

Swedenborgianism

Are you sure it runs the query twice? And, an easy suggestion would be "caching", but it doesn't answer your question. – Popliteal

My issue is not that it runs the query twice. My example doesn't work. It shows the header even though the query set is empty. – Swedenborgianism

post your view please – Vaish

Why {% if notes %} does not work ? – Esemplastic

Should it? I hoped it would, but it doesn't. I use the "if" for single variables, but it never works (for me, anyway) for query results. – Swedenborgianism

notes is a raw query set, which unlike regular query sets doesn't define a __nonzero__ method. So they evaluate to True even when empty. docs.python.org/2/reference/datamodel.html#object.__nonzero__ – Edulcorate

Answer 1

Have a look at the {% empty %} tag. Example from the documentation

<ul>
{% for athlete in athlete_list %}
    <li>{{ athlete.name }}</li>
{% empty %}
    <li>Sorry, no athletes in this list.</li>
{% endfor %}
</ul>

Link: https://docs.djangoproject.com/en/1.8/ref/templates/builtins/#for-empty

If you are interested in a table, or some kind of heading if there are results, add the forloop.first:

{% for athlete in athlete_list %} {% if forloop.first %} Athlete Name: {% endif %}
• {{ athlete.name }}
{% empty %}
• Sorry, no athletes in this list.
{% endfor %}

Quidnunc

While very useful, it does not actually answer the question. empty will run once when there are no elements. In the question, the OP wishes to display a text when there are elements in the list. (But only once) – Characteristic

@T'n'E look at my updated, which includes the lists – Fossette

Answer 2

Try {% if notes.all %}. It works for me.

Chrystalchryste

Works for me too, and I prefer this to .count, as that would issue a select count(*) query whereas I think this will cache the queryset if there is one. – Pronator

Do not use queryset as a boolean value: instead of if queryset: use if queryset.exists(): Remember, that querysets are lazy, and if you use queryset as a boolean value, an inappropriate query to a database will be carried out. @Pronator approach is better – Andersen

Thanks for the solution @KishorPawar, though in the template it doesn't have the parentheses: if queryset.exists. – Sargeant

@KishorPawar It's fine to use if queryset: because the QuerySet class overrides the __bool__ magic method. See github.com/django/django/blob/…. – Mare

Yes, it is @RuneKaagaard . Though the document says to use exists(). See the explanation for bool(). docs.djangoproject.com/en/3.2/ref/models/querysets/… – Andersen

Answer 3

It's unfortunate that you're stuck using a raw query set - they're missing a lot of useful behavior.

You could convert the raw query set into a list in the view:

notes_as_list = list(notes)
return render_to_response(template, {"notes":notes_as_list},context_instance=RequestContext(request))

Then check it as a boolean in the template:

{% if notes %}
    Header
    {% for note in notes %}
        {{ note.text }}
    {% endfor %}
{% endif %}

You could also make it happen without conversions using forloop.first:

{% for note in notes %}
    {% if forloop.first %}
         Header
    {% endif %}
    {{ note.text }}
{% endfor %}

Edulcorate

The forloop.first works for me. Nice hack. The other one doesn't. It lists all the letters of the query itself: R A W Q U E R Y S E L E – Swedenborgianism

Yeah, I checked the source for make_list but overlooked that it's registered as a stringfilter, which means it converts the argument to a unicode object if it isn't already. I'll update my answer. – Edulcorate

Answer 4

In your view check whether notes is empty or not. If it is then you pass None instead:

{"notes": None}

In your template you use {% if notes %} as normal.

Roussel

This would require running it twice, no? – Swedenborgianism

what? why? you should just assign your query to a variable notes and then do something like {"notes": notes if len(notes) else None} and you can then check if notes is None in your templates like Simeon suggests. This is the proper way to do this. – Lactescent

I think its better to do notes.count() - last I checked len(notes) pulls every record from the database whereas count simply issues a count query. – Quixotic

Answer 5

What about:

{% if notes != None %}
    {% if notes %}
        NOTES:
        {% for note in notes %}
            {{ note.text }}  
        {% endfor  %}
    {% endif %}
{% else %}
    NO NOTES AT ALL
{% endif %}

Sumption

Nope. It still prints the header (but no notes) – Swedenborgianism

Answer 6

Your original solution

{% if notes %}
    Header
    {% for note in notes %}
        {{ note.text }}
    {% endfor %}
{% endif %}

Works now with Django 1.7 and thanks to QuerySet caching, it does not cost and extra query.

Broadcloth

Answer 7

Often the right way to do this is to use the {% with ... %} tag. This caches the query so it runs only once and also gives you more flexibility with your markup than using {% empty %}.

{% with notes as my_notes %}
{% if my_notes %}
<ul>
  {% for note in my_notes %}
  <li>{{ note }}</li>
  {% endfor %}
</ul>
{% else %}
<p>Sorry, no notes available</p>
{% endif %}
{% endwith %}

With this particular example I'm not sure how useful it is but if you're querying Many-to-Many field, for instance, it's likely what you want to do.

Convertite

Answer 8

Use {% empty %} in django templates

{% if list_data %}
    {% for data in list_data %}
        {{ data.field_1 }}
    {% endfor %}
{% else %}
    <p>No data found!</p>
{% endif %}

We can write above code with {% empty %}.

{% for data in list_data %}
    {{ data.field_1 }}
{% empty %}
    <p>No data found!</p>
{% endfor %}

Unbind

Answer 9

you can use the {% empty %} for a "for loop" as in:

{% for object in list %}
<p>{{ object }}</p>
{% empty %}
<p>list is empty</p>
{% endfor %}

use '.all'

{% if object.something.all %}

Radiography