How to define custom access check function to see if a user may create a node in Drupal 7?

Question

I have several things I want to check before a user may create a node. So, if the user visits node/add/proposal I want to check if he may do so, so I wrote a module:

function proposals_menu() {

    $items['node/add/proposal'] = array(
        'title' => t('Proposal'),
        'access callback' => 'proposals_access',
    );

    return $items;
}

function proposals_access() {
    $cond1;
    $cond2;
    ...

    return cond1 && cond2 && ....;
}

When I click on add content -> proposal I get a blank page. What am I missing?

Answer 1

To override existing menu items you need to use hook_menu_alter() instead of hook_menu(). e.g.

function proposals_menu_alter(&$items) {
  $items['node/add/proposal']['access callback'] = 'some_function';
}

But there's also hook_node_access() which would be preferable to use for (as the name suggests) checking node access. e.g.

function proposals_node_access($node, $op, $account) {
  $type = is_string($node) ? $node : $node->type;

  if ($type == 'proposal' && $op == 'create') {
    if ($allow_access) {
      return NODE_ACCESS_ALLOW;
    }
    else {
      return NODE_ACCESS_DENY;
    }
  }
  return NODE_ACCESS_IGNORE;
}

Assuming you populate $allow_access with your access check. Be sure to use the $account object that's passed to the hook to verify the operation against that user object. Don't depend on the currently logged in user, which will not always be the same.

Answer 2

You get a blank page because you are not telling drupal how to render the path you are creating. For that you need to add a page callback to the item. As specified in the hook_menu documentation, this function will be called to display the page when the user visits the path.

 ...
$items['node/add/proposal'] = array(
        ...
        'page callback' => 'proposals_display_function'
    );
 ...