Merging Unrelated Git Histories - A Simple Guide

4 September 2018

Are you stuck with two Git repositories or branches that have completely different histories, but you need to merge them? You may be hitting a roadblock because Git is designed to prevent this kind of operation by default. However, there’s a workaround for this, and it’s simpler than you might think.

The Problem: Unrelated Git Histories #

Imagine you’re working on a project where you have a main branch, and someone else has a completely separate project with its own history. Now, you want to combine both projects into a single repository. If you try to perform a regular git merge or git rebase, Git will likely stop you with an error message, something like:

fatal: refusing to merge unrelated histories

The Solution: Allow Unrelated Histories #

The key to resolving this issue lies in the --allow-unrelated-histories flag. This option tells Git to overlook the fact that the two branches have no common base and to go ahead with the merge.

Here’s a simple step-by-step guide:

Fetch the Other Repository: If you’re working with a separate repository, you first need to fetch it into your current repository. You can do this with:
```
git remote add other_repo [URL_of_other_repo]
git fetch other_repo
```
Switch to the Target Branch: Make sure you’re on the branch into which you want to merge the unrelated history. Usually, this would be your main branch:
```
git checkout main
```
Perform the Merge: Now, perform the actual merge with the --allow-unrelated-histories flag:
```
git merge other_repo/other_branch --allow-unrelated-histories
```
Resolve Conflicts: If there are any file conflicts, resolve them as you usually would.
Commit and Push: Finally, commit the changes and push them to your repository:
```
git commit -m "Merged unrelated histories"
git push origin main
```

And voila! You have successfully merged two unrelated Git histories.

While it’s rare to need to merge unrelated Git histories, it’s good to know that you have the tools to get the job done when needed. Remember, use the --allow-unrelated-histories flag wisely and carefully, as it can make significant changes to your Git history.