Apply merge

Learning objectives

• practice doing Pull Requests using the GitHub interface
• practice doing a code review
• practice fixing merge conflicts on GitHub
• practice merging branches using the command-line interface
• practice fixing merge conflicts on local computer, using the command-line interface

For teachers

Teaching goals are:

• Learners have practiced doing Pull Requests using the GitHub interface
• Learners have practiced doing a code review
• Learners have practiced fixing merge conflicts on GitHub
• Learners have practiced merging branches using the command-line interface
• Learners have practiced fixing merge conflicts on local computer, using the command-line interface

gantt
  title Lesson plan apply merge
  dateFormat X
  axisFormat %s
  Introduction: intro, 0, 5s
  Theory 1: theory_1, after intro, 5s
  Exercise 1: crit, exercise_1, after theory_1, 40s
  Feedback 1: feedback_1, after exercise_1, 10s

Prior questions:

• What does a merge do?
• What does a merge do?
• Do we need merging? When? Why?
• When does a merge give a merge conflict?
• Can a git commit result in a merge conflict? Why?
• Can a git push result in a merge conflict? Why?
• Can a git pull result in a merge conflict? Why?

Branches, merging, code reviews

Branches allow us to work independently. Here we use branches to do so.

However, when we merge branches, it may result in a merge conflict. A merge conflict occurs when git is unsure how to merge branches and asks a human for help. Here we create merge conflicts on trivial code.

One can suggest to merge branches on GitHub, where it is called a Pull Request. For a Pull Request, a team member can be asked for a code review. Code reviews are useful for many reasons, among others the spread of knowledge.

One can merge branches locally, using the command-line. This will bypass code review and that is OK. For example, merging develop to your topic branch does not need a code review.

Exercises

Exercises 1 and 2 use the GitHub interface, which is graphical and easy to use. It should help you get acquainted to branches, Pull Requests and code review.

Exercises 3 and 4 use the command line instead to achieve similar goals. It should help you get acquainted to working with git on the command-line.

Exercise 5 is a repeat of doing a code review.

Exercise 1: practice merging git branches using the GitHub interface

Learning objectives

• practice merging git branches without a merge conflict

gitGraph
    commit id: "Stuff on main"
    branch develop
    checkout develop
    commit id: "Stuff on develop"
    branch anna
    checkout anna
    commit id: "Some work"
    commit id: "Branching version"
    branch bertil
    checkout bertil
    commit id: "Modify my file"
    checkout anna
    merge bertil
    commit id: "Another commit"
    checkout develop
    merge anna

• You work in a pair or trio
• On GitHub, create a branch for person A, e.g. anna that branches off from develop
• On GitHub, use the branch of person A and create a new commit. Use the web interface or command-line.
• On GitHub, create a branch for person B, e.g. bertil that branches off from anna
• On GitHub, use the branch of person B and create a new commit. Use the web interface or command-line.
• On GitHub, use web interface to create a Pull Request from bertil to anna. The person that does this requests a reviewer.
• On GitHub, the other person approves the Pull Request and merges
• On GitHub, use web interface to create a Pull Request from anna to develop. The person that does this requests a reviewer. If there is a merge conflict, either stop (you've done the exercise, well done!) or fix the merge conflict
• On GitHub, the other person approves the Pull Request and merges

Exercise 2: practice merging git branches using the GitHub interface

Learning objectives

• practice merging git branches with a merge conflict using the GitHub interface

gitGraph
    commit id: "Stuff on main"
    branch develop
    checkout develop
    commit id: "Stuff on develop"
    branch anna
    branch bertil
    checkout anna
    commit id: "Modify a file"
    checkout bertil
    commit id: "Modify the same file"
    checkout anna
    merge bertil
    checkout develop
    merge anna

• You work in a pair or trio
• On GitHub, create a branch for person A, e.g. anna that branches off from develop
• On GitHub, create a branch for person B, e.g. bertil that branches off from anna
• On GitHub, use the branch of person A and create a new commit in a file Use the web interface or command-line.
• On GitHub, use the branch of person B and create a new commit in the same place in the same (now outdated) file Use the web interface or command-line.
• On GitHub, use web interface to create a Pull Request from bertil to anna. The person that does this requests a reviewer. There is a warning for a merge conflict!
• On GitHub, the other person fixes the merge conflict, approves the Pull Request and merges
• On GitHub, use web interface to create a Pull Request from anna to develop. The person that does this requests a reviewer.
• On GitHub, the other person approves the Pull Request and merges

Extra exercise 3: practice merging git branches using the command-line

Learning objectives

• practice merging git branches without a merge conflict

Here we use the main branch for now

Instead of updating this exercise, its answer and video to use a proper branching workflow, we branch from main in this exercise

gitGraph
    commit id: "Before start"
    commit id: "Branching version"
    branch sven
    checkout sven
    commit id: "Modify my file"
    checkout main
    merge sven
    commit id: "Another commit"

• For our GitHub repo, create a branch with your first name that is unique, e.g. sven, sven_svensson or sven_svensson_314. You may branch of from main or develop (if it exists). You may use the web interface (easiest!) or use the command line
• On your local computer:
  • update your repository
  • switch to that branch
  • change the repo
  • push your changes online
• Verify the changes are online
• On your local computer
  • switch to the main branch
  • merge your topic branch to main
  • upload your changes
• Delete your topic branch (i.e. the one with the unique name). You may use the web interface (easiest!) or use the command line
• On your local computer, update your code

Answers

Click on 1, type your branch name at 2 (in this case, richel), then click 3. Done!

• On your local computer:
• update the repository

On your local computer, navigate to the folder of the shared project and update:

git pull

• On your local computer:
  • switch to the new branch

Switch to the new branch, for example, richel, by doing:

git checkout richel

• On your local computer:
  • change the content of the repository, for example, by creating a file in learners/[your_name]/[your_name]_is_on_[your_branch_name]

This can be any change you'd like. To create a file under Linux (and maybe this works on other operating systems too), one can do:

touch learners/richel/richel_is_on_richel.txt

After the change, commit these:

git add .
git commit -m "Richel is on richel"

• On your local computer:
  • push your changes online.

Do:

git push

And your code may end up online.

If that does not work, do:

git pull

and try pushing again, maybe multiple times, as many people are pushing to the shared repo.

• On GitHub, verify that your changes on your branch can be found online

Make sure you look at the correct branch, as displayed at 1. Then your commit message shows up at 2.

• On your local computer
  • switch to the main branch

git checkout main

• On your local computer
  • merge your topic branch to main

git merge richel

• On your local computer
  • upload your changes

git push

• Delete your branch (i.e. the one with the unique name). You may use the web interface (easiest!) or use the command line

Click on 'Branches', as shown in the image above.

Click on garbage bin, as shown in the image above.

The branch will now be deleted, as shown in the image above.

• On your local computer, update your code

Do:

git pull

Prefer a video?

You can find a video here

Extra exercise 4: practice merge conflicts between branches using the command line

Learning objectives

• experience merge conflicts between branches
• fix merge conflicts between branches

Here we use the main branch for now

Instead of updating this exercise, its answer and video to use a proper branching workflow, we branch from main in this exercise

gitGraph
    commit id: "Before start"
    commit id: "Branching version"
    branch anna
    checkout anna
    commit id: "Modify the file"
    checkout main
    checkout main
    commit id: "Modify the file too"
    checkout main
    merge anna
    commit id: "End"

An example picture of how to create a merge conflict.

• Create a merge conflict between two branches, e.g. a topic branch and the main branch. You can do so by creating random commits on both branches and merge. Alternatively, the figure above shows the minimal git branching history to do so.

Answers

Here, I will replay the figure above

• in GitHub, create a branch called anna
• on your local computer, git pull, then git checkout anna
• on your local computer, modify a file, e.g. add the line Anna was here at the bottom of README.md. Then do git add ., git commit -m "Anna was here" and git push. Do not merge braches yet, else there will be no merge conflicts!
• on your local computer, git checkout main
• on your local computer, modify a file, e.g. add the line main person was here. Then do git add ., git commit -m "main person was here" and git push.
• To generate the merge conflict, merge anna into main, using git merge main. You will get a clear error :-)
• Modify the file to have the texts merged. Then do git add ., git commit -m "Fixed merge conflict" and git push.

Enjoy a video?

You can find a video here

Extra exercise 5: practice code review

For team member A:

• Create a topic branch (from develop)
• Do something trivial on that topic branch
• Submit a GitHub Pull Request to merge it to develop
• Assign the other team member as the reviewer
• Do a pretend code review until the Pull Request is accepted
• The reviewer merges the code to develop

Now do the same for team member B.

Enjoy a video?

You can find a video here

Links

• Book 'Pro Git', chapter 'git branching'