Prevent post from being published if custom fields not filled

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I have a custom post type Event that contains a starting and ending date/times custom fields (as metaboxes in the post edit screen).

I would like to make sure that an Event cannot get published (or scheduled) without the dates being filled, as that will cause problems with the templates displaying the Event data (besides the fact that it is a necessary requirement!). However, I would like to be able to have Draft events that do not contain a valid date while they are in preparation.

I was thinking of hooking save_post to do the checking, but how can I prevent the status change from happening?

EDIT1: This is the hook I’m using now to save the post_meta.

// Save the Metabox Data
function ep_eventposts_save_meta( $post_id, $post ) {

if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
    return;

if ( !isset( $_POST['ep_eventposts_nonce'] ) )
    return;

if ( !wp_verify_nonce( $_POST['ep_eventposts_nonce'], plugin_basename( __FILE__ ) ) )
    return;

// Is the user allowed to edit the post or page?
if ( !current_user_can( 'edit_post', $post->ID ) )
    return;

// OK, we're authenticated: we need to find and save the data
// We'll put it into an array to make it easier to loop though

//debug
//print_r($_POST);

$metabox_ids = array( '_start', '_end' );

foreach ($metabox_ids as $key ) {
    $events_meta[$key . '_date'] = $_POST[$key . '_date'];
    $events_meta[$key . '_time'] = $_POST[$key . '_time'];
    $events_meta[$key . '_timestamp'] = $events_meta[$key . '_date'] . ' ' . $events_meta[$key . '_time'];
}

$events_meta['_location'] = $_POST['_location'];

if (array_key_exists('_end_timestamp', $_POST))
    $events_meta['_all_day'] = $_POST['_all_day'];

// Add values of $events_meta as custom fields

foreach ( $events_meta as $key => $value ) { // Cycle through the $events_meta array!
    if ( $post->post_type == 'revision' ) return; // Don't store custom data twice
    $value = implode( ',', (array)$value ); // If $value is an array, make it a CSV (unlikely)
    if ( get_post_meta( $post->ID, $key, FALSE ) ) { // If the custom field already has a value
        update_post_meta( $post->ID, $key, $value );
    } else { // If the custom field doesn't have a value
        add_post_meta( $post->ID, $key, $value );
    }
    if ( !$value ) 
                delete_post_meta( $post->ID, $key ); // Delete if blank
}

}

add_action( 'save_post', 'ep_eventposts_save_meta', 1, 2 );

EDIT2: and this is what I’m trying to use to check the post data after saving to the database.

add_action( 'save_post', 'ep_eventposts_check_meta', 99, 2 );
function ep_eventposts_check_meta( $post_id, $post ) {
//check that metadata is complete when a post is published
//print_r($_POST);

if ( $_POST['post_status'] == 'publish' ) {

    $custom = get_post_custom($post_id);

    //make sure both dates are filled
    if ( !array_key_exists('_start_timestamp', $custom ) || !array_key_exists('_end_timestamp', $custom )) {
        $post->post_status="draft";
        wp_update_post($post);

    }
    //make sure start < end
    elseif ( $custom['_start_timestamp'] > $custom['_end_timestamp'] ) {
        $post->post_status="draft";
        wp_update_post($post);
    }
    else {
        return;
    }
}
}

The main issue with this is a problem that was actually described in another question: using wp_update_post() within a save_post hook triggers an infinite loop.

EDIT3: I figured a way to do it, by hooking wp_insert_post_data instead of save_post. The only problem is that now the post_status is reverted, but now a misleading message saying “Post published” shows up (by adding &message=6 to the redirected URL), but the status is set to Draft.

add_filter( 'wp_insert_post_data', 'ep_eventposts_check_meta', 99, 2 );
function ep_eventposts_check_meta( $data, $postarr ) {
//check that metadata is complete when a post is published, otherwise revert to draft
if ( $data['post_type'] != 'event' ) {
    return $data;
}
if ( $postarr['post_status'] == 'publish' ) {
    $custom = get_post_custom($postarr['ID']);

    //make sure both dates are filled
    if ( !array_key_exists('_start_timestamp', $custom ) || !array_key_exists('_end_timestamp', $custom )) {
        $data['post_status'] = 'draft';
    }
    //make sure start < end
    elseif ( $custom['_start_timestamp'] > $custom['_end_timestamp'] ) {
        $data['post_status'] = 'draft';
    }
    //everything fine!
    else {
        return $data;
    }
}

return $data;
}

5

As m0r7if3r pointed out, there is no way of preventing a post from being published using the save_post hook, since the by the time that hook is fired, the post is already saved. The following, however, will allow you to revert the status without using wp_insert_post_data and without causing an infinite loop.

The following is not tested, but should work.

<?php
add_action('save_post', 'my_save_post');
function my_save_post($post_id) {
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
         return;

    if ( !isset( $_POST['ep_eventposts_nonce'] ) )
         return;

    if ( !wp_verify_nonce( $_POST['ep_eventposts_nonce'], plugin_basename( __FILE__ ) ) )
         return;

    // Is the user allowed to edit the post or page?
     if ( !current_user_can( 'edit_post', $post->ID ) )
         return;

   // Now perform checks to validate your data. 
   // Note custom fields (different from data in custom metaboxes!) 
   // will already have been saved.
    $prevent_publish= false;//Set to true if data was invalid.
    if ($prevent_publish) {
        // unhook this function to prevent indefinite loop
        remove_action('save_post', 'my_save_post');

        // update the post to change post status
        wp_update_post(array('ID' => $post_id, 'post_status' => 'draft'));

        // re-hook this function again
        add_action('save_post', 'my_save_post');
    }
}
?>

I’ve not checked, but looking at the code, the feedback message will display the incorrect message that the post was published. This is because WordPress redirects us to an url where the message variable is now incorrect.

To change it, we can use the redirect_post_location filter:

add_filter('redirect_post_location','my_redirect_location',10,2);
function my_redirect_location($location,$post_id){
    //If post was published...
    if (isset($_POST['publish'])){
        //obtain current post status
        $status = get_post_status( $post_id );

        //The post was 'published', but if it is still a draft, display draft message (10).
        if($status=='draft')
            $location = add_query_arg('message', 10, $location);
    }

    return $location;
}

To summarise the above redirect filter: If a post is set to be published, but is still a draft then we alter the message accordingly (which is message=10). Again, this is untested, but should work. The Codex of the add_query_arg suggests that when a variable is already it set, the function replaces it (but as I say, I haven’t tested this).

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