A simple GitHub action that will change the assignees of a PR in four situations:

1. A new PR is opened or a previously closed PR is reopened
   • If there are any review requests, the users associated with the review requests will be assigned. All other users will be unassigned.
   • If there are not any review requests, the PR creator will be assigned. All other users will be unassigned.
2. A review is requested from a user
   • All users for which there are outstanding review requests will be assigned to the PR.
   • Users who do not have a pending review request (including the PR creator) will be unassigned.
3. A review is submitted by a user
   • If the review was an "approval"
     • If there are other are outstanding review requests, the associated users will be assigned to the PR
     • Otherwise, the PR creator will be assigned
     • Users who are not the PR creator and do not have a pending review request will be unassigned.
   • If the review was not an "approval"
     • All other users for which there are outstanding review requests will be assigned to the PR; the PR creator will be assigned to the PR.
     • Users who are not the PR creator and do not have a pending review request will be unassigned.
4. A pending review request is removed
   • If there are other outstanding review requests, the users associated with the outstanding reviews will be assigned. All other users will be unassigned.
   • If there are not any outstanding review requests, the PR creator will be assigned. All other users will be unassigned.

Required. This input should contain a Github token with enough permissions to fetch a PR by number, and to change the assignees of a PR / issue.

Optional. This input may contain a JSON-stringified record with key/value pairs of usernames. All PRs created by a key username will be assigned to the value username in cases where the PR creator would otherwise be assigned.

For example, a pr-creator-assignee-substitutions value of "{ "dependabot[bot]": "dev123" }" will assign dev123 to all PRs where dependabot would otherwise have been assigned.

This assignee mapping does not affect the assignment of requested reviewers.

name: Manage assignees based on review request / submission
on:
    pull_request:
        types: [opened, reopened, review_requested, review_request_removed]
    pull_request_review:
        types: [submitted]

jobs:
    assign:
        runs-on: ubuntu-latest
        steps:
            - uses: evdhiggins/manage-pr-assignees@v1
              with:
                  # A github access token with adequate permissions to fetch a PR by number and to change its assignees.
                  token: ${{ secrets.GITHUB_TOKEN }}
                  # Assign all PRs created by dependabot to evdhiggins
                  pr-creator-assignee-substitutions: '{ "dependabot[bot]": "evdhiggins" }'

If you'd like for this action to only change the assignees for a subset of the supported situations, the triggering events / types can be adjusted to only include the desired situations.

e.g. only change assignees when a review is requested or submitted:

name: Manage assignees based on review request / submission
on:
    pull_request:
        types: [review_requested]
    pull_request_review:
        types: [submitted]