Can I do a subtree merge from a subdirectory?

Question

Let's assume two git repositories with some files in them:

ProjectA
+ a
+ b

ProjectB
+ foo/x
+ foo/y
+ bar/z

Now I want to include the directory foo from ProjectB into ProjectA.

As far as I understand when I do git subtree add the prefix is the path that it should have in the receiving repository, so when I do

git subtree add --prefix=project_b --squash URL_of_ProjectB

I would end up with

ProjectA
+ a
+ b
+ project_b
  + foo/x
  + foo/y
  + bar/z

Can I somehow specifiy that I only want foo fetched to get this layout?

ProjectA
+ a
+ b
+ project_b
  + x
  + y

Answer 1

I think this can't be accomplished with git subtree. Maybe git filter-branch is what you need:

git remote add b <URL_TO_B>
git fetch b
git checkout -b b_master b/master

Rewrite the history of b only containing foo:

git filter-branch --prune-empty --subdirectory-filter foo b_master

or use git subtree split

git subtree split -P foo --branch foo

and add it as as subtree

git subtree add --prefix=project_b --squash b_master or foo