How to include part of another git repository using git subtree and merge updates in both directions

Question

I have two git repositories show below. The first is structured like a typical python project.

foo_repo/
    .git/
    setup.py
    foo/
         __init__.py
         some_code.py
    tests/

bar/
    .git/

I would like to include the foo_repo/foo/ directory in bar/ as a subtree and I want to be able to merge updates to foo_repo/foo/some_code.py both from the foo_repo repository to bar and vice versa.

The initial setup isn't too bad. From the foo/ directory I use:

git subtree --prefix=foo/ split -b export

Then I have a new branch in foo_repo with only the contents of the foo_repo/foo/ directory. To bring this into bar, I just go to the bar/ directory and:

git subtree --prefix=foo/ add ../foo_repo/.git export

Now that I'm all set up, I'd like to do some code development and keep foo/ up to date in both repos. Pushing from bar I think I have figured out. From bar/ directory:

touch foo/more_code.py
git add foo/more_code.py
git commit -m "more code"
git subtree --prefix=foo/ push ../foo_repo/.git export

Then from the foo_repo/ directory:

git checkout master
git subtree --prefix=foo/ merge export

Merging the other way is where I'm stuck. From foo_repo/:

git checkout master
touch foo/yet_more_code.py
git add foo/yet_more_code.py
git commit -m "yet more code"
???

Where the ??? is a command that merges the foo/ directory with the export branch. Then from bar/:

git subtree --prefix=foo/ pull ../foo_repo/.git export

So I'm basically looking for the line that goes in the ??? spot, or a different workflow that does the same thing. I've tried repeating git subtree --prefix=foo/ split -b export_foo by that doesn't work.

Answer 1

If you split again foo/ the new commits created for this subtree will be created on top of the old commits already existing in export, including the merges that are needed for such:

git subtree --prefix=foo/ split

Notice that if using the -b option the branch should not exist previuosly, so you might need to create a new branch or later force it to change.