GitLab API: Get branch associated with issue - gitlab

Question

In a GitLab issue, you can associate a branch with an issue, and in the issue there will be the line
#whoever created the branch branchname to address this issue.
Is there a way of getting that branch using the issues API? I'm trying to set up an automation script that will merge all branches associated with issues that have a certain label into the prod branch, then push the result as a development brnach so I can deploy that to a dev environment. I don't want to use merge requests as they will be used when the dev work is complete and ready to be merged for deployment to production.

Answer 1

Unfortunately, there currently is no official API to fetch an issue's related branches.
Some possible ways you can work around this:
Use the notes API
When a user uses the issue interface to create the branch, you will see a system message, like you mention. This message will be present in the notes API for the issue.
Example using the python-gitlab library:
import re
...
branch_note_pattern = '^created branch \[\`(.*)\`\].*to address this issue'
issue = project.issues.get(ISSUE_NUMBER)
all_notes = list(issue.notes.list(as_list=False))
system_notes = [note for note in all_notes if note.system]
related_branches = []
for note in system_notes:
match = re.match(branch_note_pattern, note.body):
if match:
branch = match.groups()[0]
related_branches.append(branch)
print('BRANCHES RELATED TO ISSUE', ISSUE_NUMBER)
for branch_name in related_branches:
print(branch_name)
However, it is possible to have a related branch without that note appearing because the related branches is just based on naming convention. So, if someone just creates a branch with named like <issue_number>-some-branch-name then it will show up as a related branch, but there will not be a system message in the API.
So, if you rely on the notes API, you may miss related branches created manually.
Use the unofficial frontend API
The issues controller only returns related branches for the purposes of the frontend to render as HTML.
If you request /<:project_url>/-/issues/<:issue_number>/related_branches?format=json you will get a JSON response containing the HTML for the frontend to insert in the issue view. You can parse this HTML to get the related branches.
This will reliably fetch the same related branches you'll see in the UI, but is more work to implement, and is fragile because the API is not guaranteed to be stable between versions of GitLab.