Migrating azure-pipelines.yaml to separate repo, but running on code of other repo - azure

Question

Ultimately, I'm trying to do this:
Move azure-pipelines.yaml and associated templates out of the code repository (code-repo).
Move them into a separate dedicated repository (pipeline-repo).
Have the pipeline look at the config for the pipeline in pipeline-repo, but run the pipeline on the code in the code-repo.
I'm referring the following documentation:
Use other repositories: this one refers to "templates in other repositories," but I'm trying to remove any pipeline configs so the code-repo is just purely application code... and the Dockerfile.
Define a repositories resource
For testing, I have this simple test.yaml:
# Triggers when PR is created due to branch policies
trigger: none
resources:
repositories:
- repository: code-repo
type: git
name: code-repo
pool:
vmImage: 'ubuntu-latest'
stages:
- stage: Testing
displayName: Test stage
jobs:
- job: ParallelA
steps:
- bash: echo Hello from parallel job A
displayName: 'Run a one-line script'
When I create a PR on code-repo, it is triggering the pipeline, which is to say branch policies are configured to refer to that pipeline. I do get the print out the Hello from parallel job A print out.
But I don't see in the run logs it pulling code-repo.
I do see the following, however:
My actual PR pipeline would look something like this:
trigger: none
resources:
repositories:
- repository: code-repo
type: git
name: code-repo
variables:
- template: templates/variables.yaml
pool:
vmIMage: $(vmImageName)
stages:
- template: templates/build/buildStage.yaml
...
Testing that, it confirms that it isn't running on the code-repo PR, but the pipeline-repo so everything fails.
So it is unclear to me what I need to do from here to get the pipeline to run on the PR code from code-repo.
Suggestions?

Answer 1

Ok, I think I have it sorted out, at least some of my stages are now succeeding.
I came across this documentation which informed me of checkout.
So in addition to doing something like:
resources:
repositories:
- repository: code-repo
type: git
name: code-repo
Then you need to add a step called checkout like the following:
# Triggers when PR is created due to branch policies
trigger: none
resources:
repositories:
- repository: code-repo
type: git
name: code-repo
pool:
vmImage: 'ubuntu-latest'
stages:
- stage: Testing
displayName: Test stage
jobs:
- job: ParallelA
steps:
- checkout: code-repo
- task: task1
- task: task2
The checkout should set the context for the subsequent steps.