Conditions

if, elif, else

The {% if %} tag evaluates a variable, and if that variable is "true" (i.e. exists, is not empty, and is not a false boolean value) the contents of the block are output:

{% if order %}
  INSERT INTO orders (amount, user_id)
    VALUES (
      {{ order.amount | floatformat:2 }},
      '{{ user.id }}'
    );
{% endif %}

The {% if %} tag may take one or several {% elif %} clauses, as well as an {% else %} clause that will be displayed if all previous conditions fail.

Boolean operators

{% if %} tags may use and, or, or not to test a number of variables or to negate a given variable:

Example if there is an order and a payment:

{% if order and payment %}
  INSERT INTO orders (amount, user_id)
    VALUES (
      {{ order.amount | floatformat:2 }},
      '{{ user.id }}'
    );
{% endif %}

Example if there is a user or a payment:

{% if user or payment %}
  INSERT INTO payments (amount, user_id)
    VALUES (
      {{ payment.amount | floatformat:2 }},
      '{{ user.id }}'
    );
{% endif %}

Example if there is no user and a payment:

{% if not user and payment %}
  INSERT INTO payments (amount)
    VALUES (
      {{ payment.amount | floatformat:2 }}
    );
{% endif %}

Use of both and and or clauses within the same tag is allowed, with and having higher precedence than or.

{% if %} and {% elif %} tags may also use the operators ==, !=, <, >, <=, >=, in, not in, is, and is not.

Filters

You can also use filters in the {% if %} expression. For example:

{% if game.players|length >= 100 %}
  REFRESH MATERIALIZED VIEW games_critical;
{% endif %}

ifchanged

Check if a value has changed from the last iteration of a loop.

The {% ifchanged %} tag is used within a loop. It has two possible uses.

1. Checks its own rendered contents against its previous state if it has changed. For example, this INSERTs a list of sessions, only INSERTing it if it has changed:

   {% for session in batch.sessions %}
     {% ifchanged %}
       INSERT INTO sessions (id, user_id, ip, device) VALUES
         ('{{ session.id }}', '{{ session.user_id }}', '{{ session.ip }}', '{{ session.device }}');
     {% endifchanged %}
   {% endfor %}

2. If given one or more variables, check whether any variable has changed. For example, the following INSERTs the session only if the ip or the device has changed from the previous iteration:

   {% for session in batch.sessions %}
     {% ifchanged session.ip session.device %}
       INSERT INTO sessions (id, user_id, ip, device) VALUES
         ('{{ session.id }}', '{{ session.user_id }}', '{{ session.ip }}', '{{ session.device }}');
     {% endifchanged %}
   {% endfor %}

The {% ifchanged %} tag can also take an optional {% else %} clause that will be displayed if the value has not changed.