add-milestone-to-pull-requests.yml

name: Add milestone to pull requests
on:
  pull_request_target:
    types: [closed]
    branches:
      - master

jobs:
  add_milestone_to_merged:
    if: github.event.pull_request.merged && github.event.pull_request.milestone == null
    name: Add milestone to merged pull requests
    runs-on: ubuntu-latest
    steps:
      - name: Get project milestones
        id: milestones
        uses: actions/github-script@0.9.0
        with:
          github-token: ${{secrets.GITHUB_TOKEN}}
          script: |
            const list = await github.issues.listMilestonesForRepo({
              owner: context.repo.owner,
              repo: context.repo.repo,
              state: 'open'
            })
            // Need to manually sort because "sort by number" isn't part of the api
            // highest number first
            const milestones = list.data.sort((a,b) => (b.number - a.number))

            return milestones.length == 0 ? null : milestones[0].number
      - name: Update Pull Request
        if: steps.milestones.outputs.result != null
        uses: actions/github-script@0.9.0
        with:
          github-token: ${{secrets.GITHUB_TOKEN}}
          script: |
            // Confusingly, the issues api is used because pull requests are issues
            await github.issues.update({
              owner: context.repo.owner,
              repo: context.repo.repo,
              issue_number: ${{ github.event.pull_request.number }},
              milestone: ${{ steps.milestones.outputs.result }},
            });