Cache an entire page but escape an embed used for relationships (in case one of the related entry changes or some are removed / added)

stash caching and escape enmbed

{!-- Stash page caching https://github.com/croxton/Stash/wiki/Caching --}
{exp:stash:cache bundle="news"}

	{!-- layout used --}
	{stash:embed name="layouts:page"}

	{exp:channel:entries
		channel="news"
		status="open"
		disable="member_data|pagination"
		limit="1"
		require_entry="yes"
	}
		{!-- 404 --}
		{if no_results}
			{redirect="404"}
		{/if}

		{!-- meta data --}
		{exp:stash:set name="st_htmltitle"}{title}{/exp:stash:set}
		{exp:stash:set name="st_htmldescription" strip_tags="yes"}{exp:low_replace find="QUOTE" replace="""}{news_summary}{/exp:low_replace}{/exp:stash:set}
		{exp:stash:set name="st_currentnav"}newspress{/exp:stash:set}

		{!-- build entry variables --}
		{exp:stash:set name="st_item_id"}{entry_id}{/exp:stash:set}
		{exp:stash:set name="st_item_title"}{title}{/exp:stash:set}
		{exp:stash:set name="st_item_url"}{url_title_path="news_press/news_detail"}{/exp:stash:set}
		{exp:stash:set name="st_item_date"}{entry_date format="%F %j, %Y"}{/exp:stash:set}
		{exp:stash:set name="st_item_machinedate"}{entry_date format="%Y-%m-%d"}{/exp:stash:set}
		{exp:stash:set_list:categories name="st_item_categories" parse_tags="yes"}
			{categories show_group="1"}
				{stash:st_item_caturl}{path="news_press/news"}{/stash:st_item_caturl}
				{stash:st_item_catname}{category_name}{/stash:st_item_catname}
			{/categories}
		{/exp:stash:set_list:categories}
		{exp:stash:set name="st_item_img"}{news_img}{/exp:stash:set}
		{exp:stash:set name="st_item_img_caption"}{news_img_caption}{/exp:stash:set}
		{exp:stash:set name="st_item_img_copyright"}{news_img_copyright}{/exp:stash:set}
		{exp:stash:set name="st_item_summary"}{news_summary}{/exp:stash:set}
		{exp:stash:set name="st_item_body"}{news_body}{/exp:stash:set}
		{if news_author != ""}
			{exp:stash:set name="st_item_author"}{news_author}{/exp:stash:set}
		{if:else}
			{exp:stash:set name="st_item_author"}IPF{/exp:stash:set}
		{/if}

	{/exp:channel:entries}

	{!-- build page content --}
	{exp:stash:set name="content"}

		<main class="pagecontent">

			<p class="imgholder">{exp:ce_img:pair src="{exp:stash:st_item_img}" width="920" height="517" crop="yes|center,center" allow_scale_larger="yes" parse="inward"}<img src="{made}" alt="{exp:stash:st_item_img_caption} - &copy; {exp:stash:st_item_img_copyright}" class="fluidimg" />{/exp:ce_img:pair}</p>
			<p class="item-meta"><time datetime="{exp:stash:st_item_machinedate}">{exp:stash:st_item_date}</time> in <a href="{path='news_press/news'}">News</a></p>
			<h1 class="page-title  page-title--notop">{exp:stash:st_item_title}</h1>
			<div class="intro">
				{exp:stash:st_item_summary}
			</div>

			<div class="grid">
				<div class="pagecontent__primary  grid__unit  large-unit-8of12  wysiwyg">

					{exp:stash:st_item_body}

					<div class="iteminfo">
						<p class="iteminfo__author">Author: {exp:stash:st_item_author}</p>
						<p class="iteminfo__media">Picture: {exp:stash:st_item_img_caption} - &copy;&nbsp;{exp:stash:st_item_img_copyright}</p>
					</div>

				</div><!--
		 --><div class="pagecontent__secondary  grid__unit  large-unit-4of12">
        
					{!-- include related items module: same for all item types, just passing the father entry_id and using playa children to get related items  --}
					{!-- THIS IS THE ONE I'D LIKE TO NOT CACHE. Surrounding it with {stash:nocache} did not work for me --}
					{exp:stash:embed name="embeds:mod_relationships" stash:item_id="{exp:stash:st_item_id}"}

				</div>
			</div>

		</main>

	{/exp:stash:set}

{/exp:stash:cache}

Assuming you wrap the embed with {stash:nocache} you will find that on subsequent views of your template the value of {exp:stash:st_item_id} does not get set. You need to un-escape this variable so that it's value is cached the first time the template is viewed:

{stash:nocache}
    {exp:stash:embed name="embeds:mod_relationships" stash:item_id="{/stash:nocache}{exp:stash:st_item_id}{stash:nocache}" parse="inward"}
{/stash:nocache}

::lightbulb::

Now it makes sense. You want that variable to BE cached for this to work, of course. Otherwise that variable is not cached and thus not available on subsequent page loads. Never would have though about doing it this way, though.

Thank you for taking the time to explain this thoroughly.