Get real source branch name in VSTS Pull Request

We are using Visual Studio Team Systems with Git and Team System Build (former Build vNext). When we conduct a Pull Request, a new Build is triggered that is used to run Unit Tests and deploy to an isolated test system. To perform the deployment to an isolated system I need to get the real source branch name inside the build process. However the Build.SourceBranchName variable is always "merge",

E.g.:

Pull Request from source FOO to target BAR Build.SourceBranch is "refs/pull/1/merge" and therefore Build.SourceBranchName is "merge". But i need to somehow get "FOO" to run my Power Shell script to configure the system.

Is there a way to get the real source branch name inside a Git Pull Request inside VSTS?

Solution

There isn't any variable for this but you can create a power-shell script to get it via Rest API.

[String]$projecturi = "$env:SYSTEM_TEAMFOUNDATIONCOLLECTIONURI"
[String]$sourcebranch = "$env:BUILD_SOURCEBRANCH"
[String]$repoid = "$env:BUILD_REPOSITORY_ID"

$username="alternativeusername"
$password="alternativepassword"

$basicAuth= ("{0}:{1}"-f $username,$password)
$basicAuth=[System.Text.Encoding]::UTF8.GetBytes($basicAuth)
$basicAuth=[System.Convert]::ToBase64String($basicAuth)
$headers= @{Authorization=("Basic {0}"-f $basicAuth)}

#get pull request ID via regex
$pullrequest = "refs/pull/+(?<pullnumber>\w+?)/merge+"
if($sourcebranch -match $pullrequest){        
        $pullrequestid = $Matches.pullnumber;
    }
else { write-host "Cannot find pull request ID" }

#get pull request information via API
$url= $projecturi + "_apis/git/repositories/" + $repoid + "/pullRequests/" + $pullrequestid + "?api-version=1.0-preview.1"

Write-Host $url

$getpullrequest = Invoke-RestMethod -Uri $url -headers $headers -Method Get

#get sourcebranch and targetbranch
$sourceref = $getpullrequest.sourceRefName
$targetref = $getpullrequest.targetRefName