Stores transient in post meta

post-meta-transient.php

<?php

/***
Stores persisent data as post meta instead of using Transient API

@param	$post_id        int     post ID to store post meta for
@parm	$meta_key       str     "transient" name
@param	$update_func    str     function to update the meta key with a new value
@param	$expiration     time    when value should expire 

return  $value          array   stored value or updated value
***/

function yournamespace_get_post_transient( $post_id, $meta_key, $update_func, $expiration = null ) {

    if ( null == $expiration ) {
        $expiration = strtotime( '+7 days' );
    }

    //uncomment and request endpoint to clear cache:
    //delete_post_meta($post_id, $meta_key);

    $current_value = get_post_meta( $post_id, $meta_key, true );
    
    //this is the "cached" value
    //if the meta value is in the right format and it's not expired, we return it and we are done
    if ( is_array( $current_value ) && $current_value['expiration'] > time() ) {
        return $current_value['data'];
    }
    
    //either expired or didn't exist so let's call our "update" function
    $new_data = call_user_func( $update_func, $post_id );
    
    //store output in an array so that we can check expiration later
    $value = array(
        'expiration' => $expiration,
        'data' => $new_data
    );
    
    //save this new value to post meta
    update_post_meta( $post_id, $meta_key, $new_value );
    
    //return just the data for use in the endpoint
    return $value['data'];
}

/***
Update function example - this is the data you're actually using in your endpoint or custom field

@param	$post_id    int     post ID to store post meta for

return  $meta       array   Your value  

***/

function yournamespace_get_custom_meta( $post_id ) {
	
	$test_meta_value = get_post_meta('test_meta_value', $post_id);
	$some_other_thing = get_post_meta('some_other_thing', $post_id);
        $expensive = maybe_a_really_crazy_query($post_id);
	
	$meta = array(
		'test_thing' => $test_meta_value,
		'some_thing' => $some_other_thing,
                'expensive_thing' => $expensive
	);
	
	return $meta;
}

/***
Clear yc_collection_meta post_meta transient on post save/update
***/

function yournamespace_delete_meta_transient( $post_id, $post, $update ) {
        delete_post_meta( $post_id, 'yc_collection_meta' );
	yc_get_post_transient( $post_id, 'yc_collection_meta', 'yc_get_collection_meta' );
}

add_action( 'save_post_{your_post_type}', 'yournamespace_delete_meta_transient', 10, 3 );