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An example to filter out a subset of blobs/trees in a Git project, retain the history, and merge it into another Git project.

The task

I got this task to merge some directories and files from Project A into a subdirectory in Project B, and keep the related commit history in Project A.

To state this more clear let's assume I need to move subdir1 and file1 of the below Project A into Project B.

Project A

/
  /subdir1
  /subdir2
  /file1
  /file2

Project B

/
  /otherdirs
  /otherfiles

The result I want in project B is:

/
  /project_a
    /subdir1
    /file1
  /otherdirs
  /otherfiles

And all the commit history related to subdir1 and file1 should be kept.

Steps

Prerequisites

Install git-filter-repo from [here][git-filtere-repo].

Prepare the paths to merge in Project A

First, get a new clone of Project A. This is a destructive operation that will rewrite the history of the repository completely. So make sure you have a backup of the origin Project A.

Filtere out the trees and blobs we want to keep into a subdirectory project_a.

git checkout main
git filter-repo --path subdir1 \
                --path file1
                --to-subdirectory-filter project_a

This will rewrite this repository to contain only subdir1 and file1 under the directory project_a. If we want all files in Project A. We can just move all files into the directory project_a like this.

git checkout main
git filter-repo --to-subdirectory-filter project_a

Merge the subtree to Project B

# Add the filtered repository as a remote and fetch the objects.
git remote add -f project_a ../project_a/

# Merge the project A repository but do not commit.
git merge --allow-unrelated-histories -s ours --no-commit project_a/main

# Add the subtree project_a to the index.
git read-tree --prefix=project_a/ -u project_a/main:project_a

# Commit the merge.
git commit -m "merge files from Project A"