Write the value of a field as a css class to another field

Question

Could someone point me in the right direction to write the value of a field A to the css classes array of field B?

In my case I'm using a field collection (with text and file/image field, to make a sort of paragraph) and want to make a select field (with 4 preset options) so the node creator can choose if they want the file/image field displayed top, left, or right.

I don't really want to mess around with dynamic views or special formatters, just write the value of my select list to the class of the file/image field wrapper.

Answer 1

Here is some sample code to help you further which adds a class on a field (field_image) based on the value of another field (List field of type text) (field_alignment).

You will need to implement 2 preprocess hooks:

1. On node preprocess to retrieve the objects / values of the fields and add extra datamember which contains the class value
2. On field preprocess to retrieve the previously added extra datamember and merge it with the default classes_array part of a field

function YOURTHEME_preprocess_node(&$variables) 
{
    if (!isset($variables['field_image']) || !isset($variables['field_alignment'])) {
        return false; // if 1 of both fields is not available, don't do anything
    }

    $alignment = reset($variables['field_alignment']); // retrieve first item of array
    $variables['content']['field_image']['#extra_classes'] = array($alignment['value']); // add an extra datamember containing an array with the value of the other field
}

function YOURTHEME_preprocess_field(&$variables)
{
    $field = $variables['element'];
    if ($field['#field_name'] !== 'field_image') {
        return false; // only manipulate the image field
    }

    if (!isset($field['#extra_classes'])) {
        return false; // don't do anything if the array with extra classes is not available
    }

    $variables['classes_array'] = array_merge(
        $variables['classes_array'], $field['#extra_classes']); // merge the default field classes with the new added class
}

Update - When assigning class to field_image which sits within a field collection (named field_collection)

function YOURTHEME_preprocess_node(&$variables) 
{
    if (!isset($variables['field_collection']) || !isset($variables['field_classes'])) {
        return false; 
    }

    if (!isset($variables['content']['field_collection'])) {
        return false;
    }

    $alignment = reset($variables['field_classes']);
    foreach (element_children($variables['content']['field_collection']) as $index) {
         foreach ($variables['content']['field_collection'][$index]['entity']['field_collection_item'] as &$field_collection_item) {
              if (!isset($field_collection_item['field_image'])) {
                  continue;
              } 

              $field_collection_item['field_image']['#extra_classes'] = array($alignment['value']);
         }
    }
}