Merging Git repositories without losing history

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I’m the first to admit that merging two repositories isn’t something that you would do often. On the other hand, I’ve seen my fair share of situations where teams were keeping separate repositories for their front-end SPA application and the accompanying back-end APIs and running into issues. Even though keeping them separate may give a sense of isolation, in reality you end up coordinating teams across those repos whenever a big feature needs to introduced. And don’t get me started on the challenges getting those HTTP APIs aligned. So moving to a mono-repo isn’t that uncommon after all.

With a bit of Git magic, it’s quite easy to merge the commits of one repository into a specific folder of another without losing the Git history. Here’s a high-level overview.

  1. Install git-filter-repo using the instructions Elijah Newren prepared on his repo.
  2. As the git filter-repo command is a destructive operation, clone the source repo to a safe location
  3. Determine the directory at which the code from the source repo should end up
  4. Determine the prefix you want to use to add to all tags from the source repo
  5. Run git filter-repo
  6. Add the modified working directory as a git remote to the target repository
  7. Run a merge that allows unrelated histories

So let’s assume you have a repo named Source that you want to merge in the Target repo. Also suppose you want the files from Source to end up in Target under the directory Sources/Foo. Let’s start by creating a fresh clone of Source.

cd workspaces
git clone [email protected]:dennisdoomen/Source.git Source
cd source

The first step is to rewrite the history of that repo so that all files appear to be in the Sources/Foo directory and prefix all existing tags with foo-. Assuming our code is on the master branch, we can do that using:

git checkout master
git filter-repo --to-subdirectory-filter Sources/Foo --tag-rename :foo-

If everything worked as expected, you should now see that all files were moved to a Source/Foo directory.

Now we can move to the Target repo and add Source’s working directory as a new remote. So assuming that the Target repo is in the same workspaces:

cd ..\target
git remote add -f local ..\source

The final step is to import the commits from that new remote into the current one, ignoring the fact that they have no common history. And remember, we’re importing the master branch here.

git merge --allow-unrelated-histories local/master

Since the git filter-repo command rewrote the Source repo, we should delete that clone.

That’s it. That’s all it takes to merge repositories. And this works equally well when merging repositories that were integrated using a submodule. In fact, that’s exactly why we needed to figure this out in the first place.

So what do you think? Did you ever have to do this and found an alternative. Let me know by commenting below. Oh, and follow me at @ddoomen to get regular updates on my everlasting quest for better solutions.

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