Get an ACF custom field from an adjacent post

Question

I'm using the Wordpress plugin Advanced Custom Fields

I'm viewing the single.php page for a custom post type. At the bottom of the page I'd like to show next and previous post links that show an accompanying image. This image value is an ACF custom field. How do I get and display the next/previous posts image with links?

Currently my broken code below shows the image of the page I'm already viewing:

<?php 

    $size = "medium";

    $prevPost = get_previous_post(true); 

    $prevThumbnail = get_field('bg_image', $prevPost->ID); 
    $bgimage = wp_get_attachment_image_src( $prevThumbnail, $size );

?>

    <img src="<?php echo $bgimage[0]; ?>" />
    <?php previous_post_link( '<strong>%link</strong>' ); ?>

EDIT

As per @d79's comment I've done a vardump of $prevThumbnail and all that is returned is the below, what does this mean?

int(1046)

EDIT EDIT

OK, so I've done a vardump on the custom field bg_image and I get the same number returned:

int(1046)

However, after turning on debugging in wp_config, I also get the following error:

Notice: Trying to get property of non-object in /Applications/MAMP/htdocs/bao-agency/wp-content/themes/bao/single-work.php on line 19

This refers to this line:

$prevThumbnail = get_field('bg_image', $prevPost->ID);

This bg_image field is an image field set as an Image ID not an array in Advanced Custom Fields.

Answer 1

The notice error means that $prevPost is not a post object, and that can happen if the global $post is not set or there is not a previous post in the same category, so get_field('bg_image', $prevPost->ID) fails and retrieve the image of the current post.

To avoid that you need to check if $prevPost is an object:

<?php 

    global $post;
    $prevPost = get_previous_post(true);

    if( is_object($prevPost) ):

        $prevThumbnail = get_field('bg_image', $prevPost->ID); 
        $bgimage = wp_get_attachment_image_src( $prevThumbnail, 'medium' );

?>

    <?php if($bgimage): ?>
        <img src="<?php echo $bgimage[0]; ?>" />
    <?php endif; ?>
    <?php previous_post_link( '<strong>%link</strong>', '%title', true ); ?>

<?php endif; ?>

Notice that I added also the third argument true to previous_post_link to retrieve the link of the previous post but in the same category.