Summary of Changes

Auto-add catid property for com_xxx.categories context on onContentPrepare event.

It's already done in PlgSystemFields::onContentAfterTitle(), PlgSystemFields::onContentBeforeDisplay() and , PlgSystemFields::onContentAfterDisplay() which all use PlgSystemFields::display() method with:

        // If we have a category, set the catid field to fetch only the fields which belong to it
        if ($parts[1] === 'categories' && !isset($item->catid)) {
            $item->catid = $item->id;
        }

Otherwise, FieldsHelper::getFields() doesn't apply filter.assigned_cat_ids state on fields model, it results on extra database query and actually all fields applied in this event.

Testing Instructions

Assign custom field to only specific category, insert field value into category description.

Also test database queries for blog articles layout when category description is displayed and there are category custom fields.

See 4 events triggered on the category in components/com_content/tmpl/category/blog.php, they should all use the single fields load while actually we have too because the onContentPrepare event is not loading fields with filter.assigned_cat_ids model state:

$this->category->text = $this->category->description;
$app->triggerEvent('onContentPrepare', array($this->category->extension . '.categories', &$this->category, &$this->params, 0));
$this->category->description = $this->category->text;

$results = $app->triggerEvent('onContentAfterTitle', array($this->category->extension . '.categories', &$this->category, &$this->params, 0));
$afterDisplayTitle = trim(implode("\n", $results));

$results = $app->triggerEvent('onContentBeforeDisplay', array($this->category->extension . '.categories', &$this->category, &$this->params, 0));
$beforeDisplayContent = trim(implode("\n", $results));

$results = $app->triggerEvent('onContentAfterDisplay', array($this->category->extension . '.categories', &$this->category, &$this->params, 0));
$afterDisplayContent = trim(implode("\n", $results));

Actual result BEFORE applying this Pull Request

See field value in all categories, extra database query for fields load without filter.assigned_cat_ids model state.

Expected result AFTER applying this Pull Request

Field value only in field-specific category, all events for the category text are using the single load of fields for this category.