Monorepos are great.

Monorepos let us version our back-end with our front-end, making compability easy with combined builds and deployments using a shared CI/CD pipeline. They also let us make assumptions about relative relationships between folders during development.

How to Merge Two Git Repositories into a Monorepo (without losing history)

It is easier than you think to merge two separate repositories into one.

Say you have RepositoryA and RepositoryB, which you want to merge to create RepositoryC:

• RepositoryA:

  dir

  \
      \.gitignore
      \webpack.config.js
      \tslint.json
  

• RepositoryB:

  dir

  \
      \.gitignore
      \Program.cs
  

• New RepositoryC:

  dir

  \
      \.gitignore
      \RepositoryA
          \webpack.config.js
          \tslint.json
      \RepositoryB
          \Program.cs
  

You'll notice from the above differences that we have a few steps required:

• In RepositoryA,

  • Move the contents of RepositoryA into a sub-directory RepositoryA (except .gitignore)
    • Check this into a new branch for RepositoryA
    • Note: This should show as every file being a RENAME in git.
  • Push to origin:
    • git push -u origin HEAD

• In RepositoryB,

  • Move the contents of RepositoryB into a sub-directory RepositoryB (except .gitignore)
    • Check this into a new branch for RepositoryB
    • Note: This should show as every file being a RENAME in git.
  • Push to origin:
    • git push -u origin HEAD

• In RepositoryC,

  • Initialize your new repository (maybe clone an existing empty repository)

    c:\RepositoryC>  git remote -v
    origin  https://some-example@dev.azure.com/some-example/some-example/_git/RepositoryC (fetch)
    origin  https://some-example@dev.azure.com/some-example/some-example/_git/RepositoryC (push)
    

  • Create an initial commit (maybe add a README.md file)

  • Add RepositoryA and RepositoryB as remotes

    • Run:

      c:\RepositoryC>  git remote add repository-a https://some-example@dev.azure.com/some-example/some-example/_git/RepositoryA
      c:\RepositoryC>  git remote add repository-b https://some-example@dev.azure.com/some-example/some-example/_git/RepositoryA
      

    • After:

      c:\RepositoryC>  git remote -v
      repository-a     https://some-example@dev.azure.com/some-example/some-example/_git/RepositoryA (fetch)
      repository-a     https://some-example@dev.azure.com/some-example/some-example/_git/RepositoryA (push)
      repository-b     https://some-example@dev.azure.com/some-example/some-example/_git/RepositoryB (fetch)
      repository-b     https://some-example@dev.azure.com/some-example/some-example/_git/RepositoryB (push)
      origin  https://some-example@dev.azure.com/some-example/some-example/_git/RepositoryC (fetch)
      origin  https://some-example@dev.azure.com/some-example/some-example/_git/RepositoryC (push)
      

  • Fetch the latest from these newly references remotes

    c:\RepositoryC>  git fetch repository-a
    * [new branch]      master     -> repository-a/master
    
    c:\RepositoryC>  git fetch repository-b
    * [new branch]      master     -> repository-b/master
    

  • Merge each of these new branches into your repository

    c:\RepositoryC>  git merge repository-a/master --allow-unrelated-histories
    

    • Note: You may need to manually merge and commit your merge at this point, depending on what you initial commit to RepositoryC contained.

    c:\RepositoryC>  git merge repository-b/master --allow-unrelated-histories
    

    • Note: You may need to manually merge and commit your merge at this point, depending on what you initial commit to RepositoryC contained.

    --allow-unrelated-histories is the magic sauce. Read the docs

  • At this point, you should have your new file structure containing the full history of both repositories into one. 🎉

  • Push your new remote

    c:\RepositoryC>  git push -u origin HEAD
    

  • You did it!

    🥳