How to compare two commits while ignoring merge requests?

[hatboysam]

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My team follows a common workflow:

• Branch feature-x off of master
• Work on feature-x, adding commits
• Make a PR with base=master and head=feature-x
• If master is updated by someone else, do git merge master into feature-x and keep working

On the GitHub website the diff shown seems to "ignore" the changes from the merge commit. However this is not the same for the API: https://docs.github.com/en/rest/commits/commits?apiVersion=2022-11-28#compare-two-commits

When I compare two commits using the API (in the diff format) and there is a merge commit in between the two, I get a really large diff with all of the merged-in changes as well.

How can I use the API to get the exact same diff I'd see in GitHub's UI?

[jge162]

To get the same diff that you would see in the GitHub UI when using the API, you can use the diff-tree command and pass the SHA of the feature branch and the base branch (in your case, master) as arguments. This will give you a diff that only includes the changes made in the feature branch and not the changes from any merge commits.

[jge162]

Commits API (retrieves info 1st commit) and Compare Commits API (retrieves info 2nd commit) ...

1st call
GET https://api.github.com/repos/{OWNER}/{REPO}/commits/{COMMIT_SHA1}

2nd call
GET https://api.github.com/repos/{OWNER}/{REPO}/compare/{COMMIT_SHA1_1}...{COMMIT_SHA1_2}

[hatboysam]

@jge162 I really appreciate your responses here but I think we may be talking past each other a bit! I am already using the compare API but it doesn't seem to give me what I want.

Imagine a PR that has this sequence of commits:
branch from master -> A -> B -> (merge master) -> C -> D

• When I specify the base and head commits of the PR (master and D), it gives me the same diff that GitHub shows (it excludes merge changes)
• When I specify some other commits (B and D) it gives me a much larger diff containing all of the junk from the master merge that I don't care about. I am trying to figure out how to avoid that.

[jge162]

yeah this is interesting...Comparing two commits using the Compare Commits API, you can specify "the base and head" commits for the comparison in the API endpoint.

Instead of specifying the base and head commits as the first and last commits of the pull request, "you can specify the base as the common ancestor of the head and base branches of the pull request."

You can use the git merge-base command to find the common ancestor of the two branches, and then use that as the base commit in the API call. https://git-scm.com/docs/git-merge-base

use the git log command to find the merge commit SHA1, and then use the ^ notation to specify the commit before the merge commit as the base commit in the API call. https://git-scm.com/docs/git-log

For example:

GET https://api.github.com/repos/{OWNER}/{REPO}/compare/{COMMIT_SHA1_1}...{COMMIT_SHA1_2}

so COMMIT_SHA1_1 is the SHA1 of the common ancestor of the base and head branches (or the commit before the merge commit), and COMMIT_SHA1_2 is the SHA1 of the head commit of the pull request.

This way the "API will only return the changes that were made in the pull request" and "exclude any changes made in the merge commit."

[hatboysam]

Thanks for explaining those commands! That does work if I only want to see the changes for the entire PR. But the mystery to me is how to generate something like GitHub's "since my last review" diff (B to D in my example) if there have been merge commits in the mix. This is where I can't figure out how to make the API calls match up to what the GitHub UI provides.

[jge162]

you're right, generating a "since my last review" diff that excludes merge commits can be a bit tricky, especially if you're using the API. 👍

[samatcodeapprove]

Looking at the network inspector when using github.com, I see this internal API:

https://github.com/$owner/$repo/diffs/$sha_one..$sha_two?base_sha=$base_sha&whitespace_ignored=false

So what I am looking for is the public version of that!

[jge162]

The API endpoint you described is a part of the Github's internal API, which is not publicly available. However, Github does provide a public API for accessing repository data. See documentation

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