source_dirs doesn't work in .gpr scipt - ada

Question

I've inherited an Ada/C++ project and I'm trying to use gprbuild to automate the build process (which was previously done with a set of about 12 .bat files). I'm totally new to Ada and gprbuild, but have actually made pretty good progress. I can compile the .exe's that I need, but not the library. I am not at liberty to completely share the .gpr file, but the relevant parts look like this:
[snip]
for Source_Dirs use (
"c_plus_plus_files",
"ada_files",
"..\another_project\some_other_ada_files",
"..\another_project\even_more_ada_files"
);
[snip]
for Source_Files use (
"my_ada_file.ads",
"another_ada_file.ads",
"one_more_ada_file.adb",
"c_plus_plus_file.cpp"
);
[snip]
When I run "gprbuild -P my_project.gpr" it in turn runs "gcc -c gnat5 one_more_ada_file.adb" and complains that it cannot find a certain file that one_more_ada_file.adb depends on. The dependency is in ..\another_project\even_more_ada_files, so I would expect it to be found. But if I copy the dependency into the same folder as one_more_ada_file.adb, the error goes away.
Because of how the VCS is setup and how we're sharing code between two projects, I'd much rather figure out what's wrong with how I'm using "for source_dirs use" than to keep multiple copies of all the ada files.
Again, I'm an Ada/GPS newb, so if I'm leaving out relevant information, please let me know.
Update: It appears that the specific problem isn't that source_dirs isn't doing anything at all, but that it doesn't handle having two source dirs where .ads files in one dir depend on .ads files in the other. That is, even within my "other" project above, an .ads file in some_other_ada_files that depends on an .ads file in even_more_ada_files doesn't get compiled with the gcc -c -gnat05 command when I run gprbuild (error: the file in even_more_ada_files not found), but it does get compiled if I run the gcc command by hand (or in a .bat script) with two -I flags, one for each directory.

Answer 1

When dealing with multiple projects, you should normally create a .gpr-file for each project, and let your projects depend on the other projects as needed.
Thus:
project another_project is
for Source_Dirs use
("some_other_ada_files",
"even_more_ada_files");
end another_project;
and then:
with "..\another_project\another_project.gpr"
project The_Project is
for Source_Dirs use
("c_plus_plus_files",
"ada_files");
end The_Project;