Drupal 7: Conditionally change templates based on fields filled out on node creation.

d7_conditional_column_templates.tpl.php

function themefoo_preprocess_node(&$variables, $hook) {
    $node = $variables['node'];
    $type = $variables['type'];
    $columns = 1; // Default to 1 column layout.
    /*
    * Programmatically figure out how many columns we have
    * based on which fields the user has filled out in 
    * the node form.
    *
    * I'll update this gist later to explain
    */
    if(isset($variables['field_side_block_ref'])){
      if(!empty($variables['field_side_block_ref'][0])){
        $side_blocks = $variables['field_side_block_ref'];
        $columns++;
      }else{
        $side_blocks = false;
      }
    }
    if(isset($variables['field_content_image'])){
      if(!empty($variables['field_content_image'][0])){
        $content_image = $variables['field_content_image'];
        $columns++;
      }else{
        $content_image = false;
      }
    }
    /*
    * Check for File Attachments too, but don't add a column
    */
    if(isset($variables['field_file_attachments'])){
      if(!empty($variables['field_file_attachments'][0])){
        $has_attach = $variables['field_file_attachments'];
      }else{
        $has_attach = false;
      }
    }
    /*
    * Check for Gallery content, don't add a column.
    */
    if(isset($variables['field_gallery_images'])){
      if(!empty($variables['field_gallery_images'][0])){
        $gallery_images = $variables['field_gallery_images'];
      }else{
        $gallery_images = false;
      }
    }

    /*
    * Assign theme_hook_suggestions based on the number of
    * columns in the $columns variable.
    */
    switch ($columns) {
      case 1:
        // Nothing special to do here, just use node.tpl.php
        array_push($variables['theme_hook_suggestions'], 'node__1col');
        break;
      case 2:
        //$variables['theme_hook_suggestions'][] = 'node__2col';
        array_push($variables['theme_hook_suggestions'], 'node__2col');
        break;
      case 3:
        //$variables['theme_hook_suggestions'][] = 'node__3col';
        array_push($variables['theme_hook_suggestions'], 'node__3col');
        break;
    }