使用插件建立关系的一些非常好的插件：

• ACF Relationship Field
• Posts-2-Posts

使用元盒可以使用元盒建立简单的关系：

add_action( \'admin_init\', \'add_meta_boxes\' );
function add_meta_boxes() {
    add_meta_box( \'some_metabox\', \'Movies Relationship\', \'movies_field\', \'series\' );
}

function movies_field() {
    global $post;
    $selected_movies = get_post_meta( $post->ID, \'_movies\', true );
    $all_movies = get_posts( array(
        \'post_type\' => \'movies\',
        \'numberposts\' => -1,
        \'orderby\' => \'post_title\',
        \'order\' => \'ASC\'
    ) );
    ?>
    <input type="hidden" name="movies_nonce" value="<?php echo wp_create_nonce( basename( __FILE__ ) ); ?>" />
    <table class="form-table">
    <tr valign="top"><th scope="row">
    <label for="movies">Movies</label></th>
    <td><select multiple name="movies">
    <?php foreach ( $all_movies as $movie ) : ?>
        <option value="<?php echo $movie->ID; ?>"<?php echo (in_array( $movie->ID, $selected_movies )) ? \' selected="selected"\' : \'\'; ?>><?php echo $movie->post_title; ?></option>
    <?php endforeach; ?>
    </select></td></tr>
    </table>
}

add_action( \'save_post\', \'save_movie_field\' );
function save_movie_field( $post_id ) {

    // only run this for series
    if ( \'series\' != get_post_type( $post_id ) )
        return $post_id;        

    // verify nonce
    if ( empty( $_POST[\'movies_nonce\'] ) || !wp_verify_nonce( $_POST[\'movies_nonce\'], basename( __FILE__ ) ) )
        return $post_id;

    // check autosave
    if ( defined( \'DOING_AUTOSAVE\' ) && DOING_AUTOSAVE )
        return $post_id;

    // check permissions
    if ( !current_user_can( \'edit_post\', $post_id ) )
        return $post_id;

    // save
    update_post_meta( $post_id, \'_movies\', array_map( \'intval\', $_POST[\'movies\'] ) );

}

然后，要获取电影关系作为系列文章列表：

$series = new WP_Query( array(
    \'post_type\' => \'movies\',
    \'post__in\' => get_post_meta( $series_id, \'_movies\', true ),
    \'nopaging\' => true
) );

if ( $series-> have_posts() ) { while ( $series->have_posts() ) {
    $series->the_post();
    ?>
    <li><a href="<?php the_permalink(); ?>"><?php the_title(); ></a></li>
    <?php
} }