如何在上一篇文章缩略图中修复“试图获取非对象的属性”通知

Question

I added a thumbnail for next and previous post in my Wordpress theme specifically inside the single.php file.

It works as require but it is displaying the notice:

Notice: Trying to get property of non-object in \wp-content\themes\theme\template-parts\content-footer.php on line 27

I have tried out some of the examples from similar answers in this website but they do not work for me. I removed both $prevPost->ID and $nextPost->ID but it then displays the current post thumbnail instead.

The code causing the error is below: The line of code causing the notice is the 3rd and 4th lines below:

<?php 
    $prevPost = get_previous_post(); 
    $nextPost = get_next_post(); 
    $prevthumbnail = get_the_post_thumbnail($prevPost->ID, array(50,50) );  
    $nextthumbnail = get_the_post_thumbnail($nextPost->ID, array(50,50) );  
?>

The thumbnails were called using the codes below:

<div class="uk-width-auto"><?php echo $prevthumbnail; ?></div> and <div class="uk-width-auto"><?php echo $nextthumbnail; ?></div> which both works.

The error is just a notice so it does not break the website or even appear unless wordpress debug is enabled. However, I would prefer not getting this notice so as not to cause some concern for my client.

Any idea on how to resolve this?

Answer 1

So, in WordPress - the first post in the database is NOT going to have a "previous" post, and the last post is not going to have a "next" - so this behavior is perfectly normal.

To prevent the notice, you just need to check if it exists first - I typically like to use empty to perform the check - like so:

$prevPost = get_previous_post(); 
$nextPost = get_next_post(); 
if ( ! empty( $prevPost->ID ) ) {
    $prevthumbnail = get_the_post_thumbnail($prevPost->ID, array(50,50) );  
}

if ( ! empty( $nextPost->ID ) ) {
    $nextthumbnail = get_the_post_thumbnail($nextPost->ID, array(50,50) );  
}

Note that this may have the undesirable effect of leaving $nextthumbnail and / or $prevthumbnail as undefined variables, so to combat that, I'd recommend modifying the code even further:

$prevPost = get_previous_post(); 
$nextPost = get_next_post(); 
// use a ternary to set the thumbnail if not empty, or empty string if empty
$prevthumbnail = ( empty( $prevPost->ID ) ) ? '' : get_the_post_thumbnail($prevPost->ID, array(50,50) );  
// use a ternary to set the thumbnail if not empty, or empty string if empty
$nextthumbnail = ( empty( $nextPost->ID ) ) ? '' : get_the_post_thumbnail($nextPost->ID, array(50,50) );