Git Fatal Error: ‘Refusing to Merge Unrelated Histories’

The Problem

The error message fatal: refusing to merge unrelated histories is one that you might encounter when working with Git, particularly during merges. This error indicates that Git has identified two separate project histories that you’re attempting to merge and it does not understand how they can coalesce. Understanding the reasons behind this error and knowing how to resolve it is crucial for maintaining the integrity of your codebase. Here are several solutions that you can apply.

The Causes

This specific error occurs when Git detects the absence of a common commit between two histories. Generally, it arises in scenarios such as when you try to merge two repositories that started life independently of each other, when pulling changes into a new repository with its initial commit, or after purging some history from one of the repositories. The error serves as a protective mechanism to prevent potential data loss that could happen during an unintended merge.

Solution 1: Allow Unrelated Histories During Merge

Git provides a convenient flag --allow-unrelated-histories that you can use during the merge process to override the default safety behavior and force the merge of two unrelated histories.

• Step 1: Identify the branch you intend to merge with the current branch using git branch or git remote show <remote>.
• Step 2: Execute the merge command with the flag --allow-unrelated-histories to merge the specified branch into your current branch.

Example:

# Suppose you want to merge branch 'feature-branch' into your current branch
git merge feature-branch --allow-unrelated-histories

Notes: Using the --allow-unrelated-histories flag is usually safe if you know what you’re doing. However, care should be taken as it can introduce changes that might be challenging to reconcile if the histories are vastly different.

Solution 2: Rebase Instead of Merge

Rebasing is an alternative to merging that can resolve the ‘unrelated histories’ issue by replaying your changes on top of the target branch, essentially rewriting your commit history.

• Step 1: Stash any uncommitted changes using git stash to prevent loss during the rebase process.
• Step 2: Use git rebase along with the name of the target branch to begin rebasing your current branch against it.

Example:

# Suppose you are rebasing your current branch against 'main'
git rebase main

Notes: Rebasing results in a cleaner history and can help to avoid the ‘unrelated histories’ error. However, this method requires a good understanding of the implications as it rewrites history which can be problematic for collaboration.

Solution 3: Create a Common Ancestor Commit

If uncomfortable with the previous methods, you can address this problem by creating a common ancestor commit. This method manually links the histories before attempting the merge.

• Step 1: Create a new branch that will act as a bridge between the unrelated histories.
• Step 2: Create an empty initial commit on this new branch.
• Step 3: Merge both unrelated branches into this new branch one after the other without committing directly.

Example:

# Create new branch 'common-ancestor' based on current HEAD
git checkout -b common-ancestor
# Make an empty first commit
git commit --allow-empty -m 'Creating common ancestor'
# Merge the unrelated histories one by one
git merge feature-branch --no-commit --allow-unrelated-histories
git merge main --no-commit --allow-unrelated-histories

Notes: This is a safe method when dealing with unrelated histories because it manually creates a point of parity for reference. While it’s more labor-intensive, it preserves both histories without imposing changes on them.