saving/reading custom field value does not work - no value gets POSTed

Question

I am trying to add a custom field to all my posts and of course saving/reading the value. As a starting point I took the example from the Wordpress Codex and stripped it down to bare bones. I know it know has serious security flaws, I am just trying to understand it.

The problem I am facing at the moment is that nothing is saved. After looking at what is POSTed when the post is saved, I noticed that the input's key & value is not present in the $_POST array at all. The property exists in the database, but without value of course.

I also noticed that the variable $post passed to the add_input is empty, but I don't why (I guess this is the problem?).

This is the code from my functions.php:

function add_custom_field() {
    add_meta_box( 'my_field', 'My Field', 'add_input', 'post' );
}
add_action( 'add_meta_boxes', 'add_custom_field' );

function add_input( $post ) {
    $value = get_post_meta( $post->ID, 'my_key', true );
    echo '<input type="text" value="' . $value . '" id="my_id">';
}

function save( $post_id ) {    
    $mydata = sanitize_text_field( $_POST['my_id'] );
    update_post_meta( $post->ID, 'my_key', $mydata );
}
add_action( 'save_post', 'save' );

Do you see any errors in the code above? Do you have any comments to my info I gave you? Why is nothing POSTed to the server?

Answer 1

You have two issues:

1.- Your are not passing the correct post ID. To solve this problem change:

update_post_meta( $post->ID, 'my_key', $mydata );

To:

update_post_meta( $post_id, 'my_key', $mydata );

2.- Your are not createing the input correctly, the name attribute is missed:

Change:

echo '<input type="text" value="' . $value . '" id="my_id">';

To:

echo '<input type="text" value="' . $value . '" id="my_id" name="my_id">';