Checkout not checking out to PR branch

[Lucasmiguelmac]

Let's say I have a dev target branch and I want to set up a GH Action .yml file to run on every PR.

The first step,that is, the actions/checkout@v2 step, is no longer doing what it's supposed to. Which would be: checking out to the branch I'm wanting to merge.

name: Run tests
on:
  pull_request:
    types: [opened, reopened, synchronize]
jobs:
  trigger-job:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout 
        uses: actions/checkout@v2
      - name: Print current branch
        run: echo "$(git branch)" # --> echoes "(HEAD detached at pull/<pr-number>/merge)"

For demonstration purposes:

1. I attach a side by side comparisson of the same pipeline run with a 4 days difference
   
2. I tried the same on another non related GH user and had the same error

Why is that? The same Print current branch step would print the actual name of the branch I'm trying to merge into dev a couple of days ago, but not anymore

[bangank36]

@Lucasmiguelmac by default the input ref will point to single commit
Setting fetch-depth: 0 can help pull all history and tags but it is not recommended if you have a large repo, therefore you can set the ref input like so to checkout the correct branch

name: Run tests
on:
  pull_request:
    types: [opened, reopened, synchronize]
jobs:
  trigger-job:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout 
        uses: actions/checkout@v2
        with: 
+               ref: ${{ github.head_ref }}
      - name: Print current branch
        run: echo "$(git branch)" # --> echoes "(HEAD detached at pull/<pr-number>/merge)"

You can see my action log that the correct branch has been checked out