[patch] Support CUSTOM_ID property in latex export

[Richard Lawrence <richard.lawrence@berkeley.edu>]

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From 81115d0884c165778520aa1b4d4fa83580417e1c Mon Sep 17 00:00:00 2001
From: Richard Lawrence <richard.lawrence@berkeley.edu>
Date: Sat, 15 Feb 2014 11:59:44 -0800
Subject: [PATCH] LaTeX export: support CUSTOM_ID property in section labels
 and link refs

* lisp/ox-latex.el (org-latex-headline): when exporting a headline, if
  it has a CUSTOM_ID property, use that value as the associated
  label for a section (or whatever)
(org-latex-link): when exporting a link, if the
  destination is a headline with a CUSTOM_ID property, use that value
  in the referencing command

Fixes an issue raised at: http://thread.gmane.org/gmane.emacs.orgmode/54039

TINYCHANGE
---
 lisp/ox-latex.el |   25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/lisp/ox-latex.el b/lisp/ox-latex.el
index 5815874..cbca0a5 100644
--- a/lisp/ox-latex.el
+++ b/lisp/ox-latex.el
@@ -1372,11 +1372,14 @@ holding contextual information."
 	   (full-text (funcall org-latex-format-headline-function
 			       todo todo-type priority text tags))
 	   ;; Associate \label to the headline for internal links.
+	   ;; Use the value of :CUSTOM_ID: property as label if it is defined.
 	   (headline-label
-	    (format "\\label{sec-%s}\n"
-		    (mapconcat 'number-to-string
-			       (org-export-get-headline-number headline info)
-			       "-")))
+	    (let ((custom-label (org-element-property :CUSTOM_ID headline)))
+	      (if custom-label (format "\\label{%s}\n" custom-label)
+		(format "\\label{sec-%s}\n"
+			(mapconcat 'number-to-string
+				   (org-export-get-headline-number headline info)
+				   "-")))))
 	   (pre-blanks
 	    (make-string (org-element-property :pre-blank headline) 10)))
       (if (or (not section-fmt) (org-export-low-level-p headline info))
@@ -1846,11 +1849,15 @@ INFO is a plist holding contextual information.  See
 	  ;; title.
 	  (headline
 	   (let ((label
-		  (format "sec-%s"
-			  (mapconcat
-			   'number-to-string
-			   (org-export-get-headline-number destination info)
-			   "-"))))
+		  (or
+		   ;; Case 1: headline has a CUSTOM_ID property; use that as label
+		   (org-element-property :CUSTOM_ID destination)
+		   ;; Case 2: headline has no CUSTOM_ID; use default numbering 
+		   (format "sec-%s"
+			   (mapconcat
+			    'number-to-string
+			    (org-export-get-headline-number destination info)
+			    "-")))))
 	     (if (and (plist-get info :section-numbers) (not desc))
 		 (format "\\ref{%s}" label)
 	       (format "\\hyperref[%s]{%s}" label
-- 
1.7.10.4


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Hi Orgsters,

Here is a patch to add support for using CUSTOM_ID properties for labels
and refs in the LaTeX exporter.

The patch uses the value of CUSTOM_ID when exporting a headline for the
associated \label, and when exporting a link for the associated \ref
command (or whatever).

For example, from this Org file:

===============================================================================
* Headline 1
  :PROPERTIES:
  :CUSTOM_ID: sec:headline1
  :END:
  Links to headlines which have no CUSTOM_ID still work normally, like
  this one: [[Headline 2]].

* Headline 2
  Links to headlines which have a CUSTOM_ID property will use this
  value to refer to them:

  This link refers to Headline 1 using the builtin syntax for
  CUSTOM_ID: [[#sec:headline1]].

  This one uses the fuzzy search on the headline text: [[Headline 1]].
===============================================================================

the relevant section is now exported as:

===============================================================================
\section{Headline 1}
\label{sec:headline1}
Links to headlines which have no CUSTOM\(_{\text{ID}}\) still work normally, like
this one: \ref{sec-2}.
\section{Headline 2}
\label{sec-2}
Links to headlines which have a CUSTOM\(_{\text{ID}}\) property will use this
value to refer to them:

This link refers to Headline 1 using the builtin syntax for
CUSTOM\(_{\text{ID}}\): \ref{sec:headline1}.

This one uses the fuzzy search on the headline text: \ref{sec:headline1}.
===============================================================================

Previously, the label for Headline 1 would have been \label{sec-1}, and
the links under Headline 2 would have been exported as \ref{sec-1}. 

This addresses an issue that was raised here, but got no response:
http://thread.gmane.org/gmane.emacs.orgmode/54039

I also need this behavior, which is why I wrote this.

Hope that's helpful!

Best,
Richard