Support `auto_merge` field in PullRequest struct

[fho]

The pulls API returns an auto_merge field which is currently not available in the PullRequest struct.

When auto-merge is enabled for a Pull-Request the returned auto_merge field looks like:

  "auto_merge": {
    "enabled_by": {
      "login": "fho",
      "id": 514535,
      "node_id": "MDQ6VXNlcjUxNDUzNQ==",
      "avatar_url": "https://avatars.githubusercontent.com/u/514535?v=4",
      "gravatar_id": "",
      "url": "https://api.github.com/users/fho",
      "html_url": "https://github.com/fho",
      "followers_url": "https://api.github.com/users/fho/followers",
      "following_url": "https://api.github.com/users/fho/following{/other_user}",
      "gists_url": "https://api.github.com/users/fho/gists{/gist_id}",
      "starred_url": "https://api.github.com/users/fho/starred{/owner}{/repo}",
      "subscriptions_url": "https://api.github.com/users/fho/subscriptions",
      "organizations_url": "https://api.github.com/users/fho/orgs",
      "repos_url": "https://api.github.com/users/fho/repos",
      "events_url": "https://api.github.com/users/fho/events{/privacy}",
      "received_events_url": "https://api.github.com/users/fho/received_events",
      "type": "User",
      "site_admin": false
    },
    "merge_method": "rebase",
    "commit_title": "",
    "commit_message": ""
  },

It would be great if go-github could parse this field and make it accessible.

[gmlewis]

Thank you, @fho !

This would be a great PR for any new contributor to this repo or a new Go developer.
All contributions are greatly appreciated!

Feel free to volunteer for any issue and the issue can be assigned to you so that others don't attempt to duplicate the work.

Please check out our CONTRIBUTING.md guide to get started. (In particular, please remember to go generate ./... and don't use force-push to your PRs.)

Thank you!

[slayer321]

Hey @gmlewis, I am new to opensource and Golang, I will like to contribute to this project can you guide me on what needs to be done.
I have read the CONTRIBUTING.md and I also signed the CLA.

[gmlewis]

Hi @slayer321 - welcome to Go and to open source and thank you for your interest in helping out this project!

Sure... when attacking any kind of problem, it is best to break it down into smaller, more easily understandable parts.

So for this specific task, I would basically do the following:

• Fork and clone this repo if you haven't done so already
• Make sure you can run tests locally before you attempt to make any changes

As an aside, I personally use the following test script so that I don't forget anything:

#!/bin/bash -ex
go generate ./...
go test ./...
go vet ./...

• Create a new branch for your development... I like to put the issue in the name... something like:

$ git checkout -b i1881-support-auto-merge

OK, I can see this message is going to get huge... so I'll just start commenting and keep creating more comments as I break down the problem. Stay tuned... 😂

[gmlewis]

• Next, since @fho was so kind as to provide us a URL regarding the issue, I take a look at it.
• I see that the endpoint in question is: GET /repos/{owner}/{repo}/pulls
• I now try to find out where this is in our repo's code base by searching for repos/%v/%v/pulls and making sure it is a GET method

OK, I'm getting pulled into a meeting so I'm going to have to continue to address this later today.

[gmlewis]

• I then match that endpoint with the code, in this case: https://github.com/google/go-github/blob/master/github/pulls.go#L135-L145 and check that the URL for the method is pointing to the same endpoint.
• I see that PullRequest is the struct being returned, which is located here: https://github.com/google/go-github/blob/master/github/pulls.go#L21-L77
• I then search for the "auto_merge" field which is what is currently missing... both in the docs and in the struct... and yes, I confirm that it is in the docs ("auto_merge": null) and not in the code.
• I see that the author of the issue shows an example response for this field even though the GitHub v3 API documentation does not, so I base my implementation on the provided example.
• It looks to me like the "auto_merge" field has 4 sub-fields and should go at the top-level of the PullRequest struct... and one of the fields looks very much like our existing User struct. So I would write some code like this:

// PullRequestAutoMerge represents the "auto_merge" response for a PullRequest.
type PullRequestAutoMerge struct {
    EnabledBy *User `json:"enabled_by,omitempty"`
    MergeMethod *string `json:"merge_method,omitempty"`
    CommitTitle *string `json:"commit_title,omitempty"`
    CommitMessage *string `json:"commit_message,omitempty"`
}

Then I would add this new struct into the PullRequest struct, probably after RequestedReviewers:

...
	RequestedReviewers  []*User    `json:"requested_reviewers,omitempty"`
	AutoMerge *PullRequestAutoMerge `json:"auto_merge,omitempty"`
...

Afterward, I would ideally incorporate this new value in an existing or new unit test (following examples in the surrounding code)... although it is not always necessary to do this just when adding new fields. It is a nice-to-have to make sure that the struct gets populated properly from JSON... and you could use the example given by the author of the issue in this case.

Finally, I would test out all my changes using that same script I shared above and make sure everything is working properly, then I would share my PR on GitHub and ask for a code review.

I think that about summarizes things... please let me know if you have any questions, and best wishes to you.

[fho]

Wow, I did not expect such a fast response. :-)
@slayer321 thanks a lot for volunteering! Please let me know if you should change your plans to do the implementation, then I would pick it up instead
@gmlewis excellent step-by-step guide! :-)

[slayer321]

Thank you very much @gmlewis for explaining everything step by step. Most of the steps I understood and some I didn't but I am learning about it.

As you have assigned it to me I will start working on it and if I face any problems I will let you know 😃

[bharathk005]

@gmlewis Since there were no updates for around 11 days, I thought I will go ahead and give this a try. I have created a PR #1894 for the same. Please check and let me know.
Also, I feel it would be nice to have all the test files in a separate folder

[gmlewis]

Sorry, @bharathk005 , but let us give @slayer321 an opportunity to respond before taking over an issue that is already assigned.

Additionally, I looked at your PR and it has a number of problems, so I'm not going to officially review it until we hear back from @slayer321.

As for putting test files in a separate folder, that is not idiomatic Go and we will not accept that kind of change in this repo.

[bharathk005]

@gmlewis
Thanks for the quick response. Sure let's wait for @slayer321

[slayer321]

Sorry, @gmlewis for replying so late. I was not able to work on this issue as my college exam started and I was busy with that but now as the exams are over I can start working on it.