How to define a custom field by using two functions?

Attachments are typically associated with posts, even if not featured images. You can normally get all image attachments to a post by using get_posts(array('post_type' => 'attachment','post_parent' => $post_id, 'post_mime_type' => 'image/jpeg'));. You can then just grab the ID from the first object returned and use wp_get_attachment_image_src() to get the image URL.

The problem arises when a post has an image inserted from the media library that was initially used in a different post. I know of no way to query for this type of image, AFAIK, one has to parse the content for image tags and hopefully the ID is in one of the assigned classes of the tag.

If you are dynamically grabbing the first image of a post, why have it shown in a special field? Why not just use it directly for whatever the field’s purpose was? So the URL can be overridden manually if need be? If so, it should only be placed in postmeta if there is no value at all. If there were a value there, it may have been manually placed so your script must not override that.

I’m afraid I’ve raised more questions rather than answer yours. If you could more specifically explain what you want to do, perhaps I can suggest a workable direction to pursue.

Ah, I see now. So overwriting the previous meta value is not a big deal, it’ll be the same value in most cases. But I’m also confused about sub-domain images. For a thumbnail to exist, the image would have been uploaded by the media uploader. Are you saying this was done, but not relative to the WP installation with the ratings list? So all the various image sizes are available in the sub-domain, which is not the uploads folder of this installation?

How are you adding images from the sub-domain to posts? If you use the media uploader, it will copy the referenced file to uploads and regenerate all the registered sizes as well. If you manually enter an <img> tag in the post, the sub-domain image will be used, but it becomes very difficult to know what the thumbnail filename would be, because as far as this installation is concerned, it does not exist.

You would need to exactly replicate the filename logic employed when the thumbnails were created in order to properly enter the correct URL in the custom field in postmeta.

Am I misunderstanding something? Does any of this make sense?

I don’t use subdomains, but if your logic is correct, this should work:

add_action('publish_post', 'add_custom_field_automatically');
function add_custom_field_automatically($post_id) {
	$attachments = get_posts(array('post_type' => 'attachment','post_parent' => $post_id, 'post_mime_type' => 'image/jpeg'));
	$src = wp_get_attachment_image_src($attachments[0]->ID, 'thumbnail');
	add_post_meta($post_id, 'thumbnail_rt', $src[0], true);
}

I did test this and it works fine for images in /uploads/, except that (at least for the test post I used) it seems the first image in the array returned is actually the last image in the post. It may just be the images of this particular post were added in an odd order. The script may need to be modified to find the size of the array and grab the last image, a bit more testing is required.

And of course I can’t know if it will work for subdomain images. No matter what, it should be a step in the right direction.

Great news! Progress- gotta love it.

Yeah, the reused image issue is a big one, which I alluded to in my 1st post. It is true there is only one attachment per image, regardless of the number of times the image is used. As such there is no easy way to query for reused images and establish the attachment ID. The best I can come up with is search each post’s content for an <img> tag. The ID number should be contained in one of the assigned classes. Failing that, we’d have to extract the src URL and query for a matching attachment that way. Either approach strikes me as rather fragile, but I don’t have a better idea.

As for working on updates, in theory it does. Or at least the action fires and the function runs on updates. The problem is related to the reused image issue. Here is what I believe is happening, though I haven’t confirmed by actual testing. When you remove an image from the post, it remains in the image library and the attachment post remains in the DB with the post as parent, even though it does not appear in the post content. So when the function runs, it finds the first image attached instead of the one recently added for the update. Thus it appears to not work on updates, but it actually does… just not the way we want.

So once again we find ourselves needing to search content in order to be sure we find an image that is actually used in the current version of the post. The attachments are simply unreliable except when one structures their blog to only use new images and never goes back and fiddles with the images used in content.

Close but not quite. The problem is the $attachment_url = '' portion. This syntax is proper when defining a function to assign a default value to a parameter should it be omitted from the calling function. It is improper when actually calling the function because it overwrites the value you intended to pass.

Which brings us to another problem. When you call pn_get_attachment_id_from_url(), the variable $attachment_url has not been assigned a value, so the entire process serves no purpose.

But as for assigning the return value to $attachment_id, which was your actual question, that part is fine, that’s the way to do it.

Finally, w_thumbnail_src() is never called because it was not added as an action callback correctly. You cannot list multiple callback functions like that, you need a separate add_action() call for each callback you wish to add. For the position you placed the w_thumbnail_src parameter, the add_action() function is expecting a priority argument, an integer.

These are what I saw, there may be other issues. Take care of those and see if you get anywhere. Good luck!