Don't delete a page if it holds users

Question

I have to do something but I just can't come up with a way to do it.

On one hand I have created pages who are companies. So every page is a company-page.
On the other hand I have created users who have a custom field added to their profile that holds the name of the company they work at.

No this is merely a connection between those two. So I came to the conclusion that if a company is deleted the users should be to, or at least a message should appear that the company can't be deleted because there are still users that are connected to the company.

I think I should hook in some sort of function to add an action to the page-delete button.

It's almost like if you delete a user and WP asks you if the posts the user has made are assigned to another user.

Can anybody tell me more about this or share some thoughts/ideas?

===================================
UPDATE
So I created the following for a custom_function to hook in the before_delete_post
NOTE: this is tested on a company page for output purposes. I use a $_GET["company"] request because the name is passed on in the URL.

global $post;
$company = $_GET['company'];
$employees = $wpdb->get_results("
             SELECT meta_value 
             FROM $wpdb->usermeta 
             WHERE meta_key = 'company' 
             AND meta_value='$company'"
             );

/* 
output $employees: 
[0] => stdClass Object
    (
        [meta_value] => company_name
    )
*/

if(in_array($company, $employees)){
    echo 'Yes';
}else{
    echo 'No';
}

Let's say I want to delete the page company_name. What I do is see if this name is in the usermeta table. If this is true the delete function should not be executed. If not the page can be deleted.

But this just keeps echo-ing No while it should echo Yes because I know the pagename is company_name and it is in the employee list. So I should have a match.

Answer 1

It gives an array with objects. So in_array() won't find anything, you have to convert it before. After you get the results from wbdb:

$employees_array = array();
foreach($employees as $employee) {
    $employees_array[] = $employee->meta_value;
}

if(in_array($company, $employees_array)) {
    ...
}

Answer 2

this can be best solved by adding constraints in your database schemes. The best way to achieve this is by adding a foreign key constraint which reacts on the "DELETE" action. For more info about the foreign key read: http://dev.mysql.com/doc/refman/5.6/en/create-table-foreign-keys.html

You could also do a SELECT before calling the DELETE action and if it returns users exit the function with a error of some kind.