Add custom field to attachments in a specific category

Question

I have a working function that adds a meta field to all attachments (so I can enter the url of a video). I would like this field to populate only on attachments in the "video" category.

I've tried the below, by adding in_category('video') but that didn't work. The field still shows on all attachments.

function attachment_field_url( $form_fields, $post ) {

        $form_fields['video-url'] = array(
        'label' => 'Video URL',
        'input' => 'text',
        'value' => get_post_meta( $post->ID, 'video_url', true ),
        'helps' => 'Add video URL',
    );

    return $form_fields;
}

add_filter( 'attachment_fields_to_edit', 'attachment_field_url', 10, 2 );

function attachment_field_url_save( $post, $attachment ) {

    if( in_category('video')  && !isset ($attachment['video-url'] ) )
update_post_meta( $post['ID'], 'video_url', esc_url( $attachment['video-url'] ) );                             
    return $post;
}

add_filter( 'attachment_fields_to_save', 'attachment_field_url_save', 10, 2 );

Answer 1

in_category relies on a global post - it should only be used within the loop. Instead, use the arguments that are passed to your callback to query the categories assigned to the post being edited and check if one of them is video:

$cats = get_the_category( $post['ID'] );
if ( in_array( 'video', wp_list_pluck( $cats, 'slug' ) ) {
    // In "video" category
}

...altogether now:

function attachment_field_url_save( $post, $attachment ) {
    if ( isset( $attachment['video-url'] ) ) {
        $cats = get_the_category( $post['ID'] );

        if ( in_array( 'video', wp_list_pluck( $cats, 'slug' ) ) {
            update_post_meta( $post['ID'], 'video_url', esc_url( $attachment['video-url'] ) );  
        }
    }

    return $post;
}