Org-babel: Set root dir for tangled files

Question

I have started using org-babel for a larger project, and have found the readthedocs to be a great resource. I have been using tangling to generate code from documentation, however setting the export filename using :tangle is gettting cumbersome, since for each block I need to set all of the relative paths and whatnot.

I know that setting the :dir header argument allows you to choose what directory code blocks will be executed in, but it does not set the tangle directory. Ideally I would like to have something like this:

#+STARTUP: showall
#+PROPERTY: header-args :dir "/home/user/project/path1/path2/path3"

* Test

** myfile.h
:PROPERTIES:
:header-args: :tangle filename1.cpp :no-expand t
:END:
#+BEGIN_SRC C++

#+END_SRC
** myotherfile.cpp
:PROPERTIES:
:header-args: :tangle filename2.cpp :no-expand t
:END:
#+BEGIN_SRC C++

#+END_SRC

Rather than:

#+STARTUP: showall

* Test

** myfile.h
:PROPERTIES:
:header-args: :tangle ../../path2/path3 :no-expand t
:END:
#+BEGIN_SRC C++

#+END_SRC
** myotherfile.cpp
:PROPERTIES:
:header-args: :tangle ../../filename2.cpp :no-expand t
:END:
#+BEGIN_SRC C++

#+END_SRC

Imagine how frustrating this could get if another person takes over the project, or one needs to change the directory layout? I have poked around various sources but haven't found a way to do this. This question is related, but doesn't solve my issue

Answer 1

I had this problem and I solved by executing elisp code in tangle setting

* Project Header
:PROPERTIES:
:PRJ-DIR: ~/prj/dir/
:END:

#+BEGIN_SRC js :tangle (concat (org-entry-get nil "PRJ-DIR" t) "filename.js")
  var a = 2;
#+END_SRC