Display Repeater sub-field, a post object value - repeater

Question

I have a custom post_type 'publications' and it has a Repeater field 'linked_author' with a sub-field 'paper_linked_author' - a post object.
Here is the code to get all of the publications post and run a loop to get all authors linked to each post.
$args = array(
'posts_per_page' => '-1',
'post_type' => 'publications'
);
$the_query = new WP_Query( $args );
$post_ids = wp_list_pluck( $the_query->posts, 'ID' );
echo "<pre>";
print_r($post_ids);
echo "</pre>";
$artist_list = array();
foreach($post_ids as $post_id){
$artist_repeater = get_field('linked_author', $post_id);
echo "<pre>";
print_r($artist_repeater);
echo "</pre>";
if ($artist_repeater) {
$speakers = get_sub_field('paper_linked_author');
if ($speakers && count($speakers)>0)
{
foreach ($speakers as $speaker)
{
echo $speaker->ID; //$speaker is a post object//
}
}
}
}
I have been able to get all the sub-field values of each publication post however I'm not able to echo details of each linked author - the sub-field values. A print_r($artist_repeater) shows below details for one of the posts.
So, my question is how to list all unique authors from below print_r() array? Also, I need to link each author name such that clicking will bring list of all publications post he/she is linked to? Please help!
Array
(
[0] => Array
(
[paper_linked_author] => WP_Post Object
(
[ID] => 4885
[post_author] => 1
[post_date] => 2018-06-26 04:56:29
[post_date_gmt] => 2018-06-26 04:56:29
[post_content] =>
[post_title] => Q1-Test
[post_excerpt] =>
[post_status] => publish
[comment_status] => closed
[ping_status] => closed
[post_password] =>
[post_name] => q1-test
[to_ping] =>
[pinged] =>
[post_modified] => 2018-06-26 05:14:13
[post_modified_gmt] => 2018-06-26 05:14:13
[post_content_filtered] =>
[post_parent] => 0
[menu_order] => 0
[post_type] => people
[post_mime_type] =>
[comment_count] => 0
[filter] => raw
)
)
[1] => Array
(
[paper_linked_author] => WP_Post Object
(
[ID] => 2115
[post_author] => 1
[post_date] => 2017-03-28 05:47:01
[post_date_gmt] => 2017-03-28 05:47:01
[post_content] =>
[post_title] => Julius Ceaser
[post_excerpt] =>
[post_status] => publish
[comment_status] => closed
[ping_status] => closed
[post_password] =>
[post_name] => julius-ceaser
[to_ping] =>
[pinged] =>
[post_modified] => 2018-06-25 11:02:55
[post_modified_gmt] => 2018-06-25 11:02:55
[post_content_filtered] =>
[post_parent] => 0
[menu_order] => 0
[post_type] => people
[post_mime_type] =>
[comment_count] => 0
[filter] => raw
)
)
)

Answer 1

Just so it helps others, I solved the above problem with below code.
Basically, I was looking to list all authors who have written at least one Publication. So, earlier I was querying publications custom post_type and then looping each publication to list linked authors and that is where the above print_r() dump can be seen.
So, instead I queries all authors (a custom post_type) and then queries publications to match the key "linked_author_$_paper_linked_author" with the ID of the author post. It worked.
<?php
add_filter( 'posts_orderby' , 'posts_orderby_lastname' );
$args = array(
'post_type' => 'people',
'posts_per_page' => -1, // Unlimited posts
'post_status' => 'publish'
// Order alphabetically by name
);
$loops = new WP_Query( $args );
while ($loops -> have_posts()) : $loops -> the_post();
$args = array(
'numberposts' => -1,
'post_type' => 'publications',
'meta_query' => array(
'relation' => 'AND',
array(
'key' => 'linked_author_$_paper_linked_author',
'compare' => '=',
'value' => get_the_ID()
)
)
);
// query
$the_query = new WP_Query( $args );
if( $the_query->have_posts() ):
?>
<ul>
<li><?php echo the_title(); ?></li>
</ul>
<?php
endif;
endwhile;
?>