why shouldn’t i save metadata when its a revision

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I’m reading a book about WordPress and I’m new and confused.

Why does the author always not save data in the metadata box when its a revision. just in case its not clear, what i refer to by metadata box is the one added by add_meta_box.

//save meta box data 
function pp_save_meta_box($post_id,$post) {
    // if post is a revision skip saving our meta box data
    if($post->post_type == 'revision') { return; }

    // process form data if $_POST is set
    if(isset($_POST[’pp_sku’]) && $_POST[’pp_sku’] != ‘’) {
        // save the meta box data as post meta using the post ID as a unique prefix
        update_post_meta($post_id,’pp_sku’, esc_attr($_POST[’pp_sku’]));
        update_post_meta($post_id,’pp_price’, esc_attr($_POST[’pp_price’]));
        update_post_meta($post_id,’pp_weight’, esc_attr($_POST[’pp_weight’]));
        update_post_meta($post_id,’pp_color’, esc_attr($_POST[’pp_color’]));
        update_post_meta($post_id,’pp_inventory’,esc_attr($_POST[’pp_inventory’]));
    }
}
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2 comments

  1. You omitted how is this function called. I assume it is added to save_post action.

    This action passes current post id as argument. In case of revision that would be revision id and not parent post id. So, as I see it, there is no reason to save additional data for revision (creating duplicate set of it).

    Update.

    Scratch that.

    I looked through source code. Apparently *_post_meta functions will automatically change to parent post id if passed revision post id. So you might modify original post, thinking you are modifying revision.

  2. When I look at my postmeta table, it appears no metadata is ever saved for post revisions. I don’t think it has to be this way: I can think of some use cases where it would be useful for plugins to save additional metadata for revisions too. Take your example, probably from a webshop example: if I first create a product with a price of $20, then later reduce the price to $10, this will probably save the old text fields as a revision, but it will erase the old price fields. It could be useful to save a history of the prices too.

    I assume the functions that deal with metadata don’t fully support working with revisions of metadata, so we should not expect much support from plugins yet. But, if you create a plugin, keep in mind how saving (and retrieving) historical metadata could be useful.