old_contents: from django.template.base import Library
  from django.core.urlresolvers import reverse
  
  from projects.models import ProjectBuild
  
  
  register = Library()
  
  
  @register.simple_tag()
  def build_url(build_id):
      """
      Fetches the ProjectBuild for a given build_id, if any.
      """
      try:
          build = ProjectBuild.objects.get(build_key=build_id)
          return reverse(
              "project_projectbuild_detail",
              kwargs={"project_pk": build.project.pk, "build_pk": build.pk})
      except ProjectBuild.DoesNotExist:
          return ""
  
new_contents: from django.template.base import Library
  from django.core.urlresolvers import reverse
  
  from projects.models import ProjectBuild
  
  
  register = Library()
  
  
  @register.simple_tag()
  def build_url(build_key):
      """
      Fetches the ProjectBuild for a given build_id, if any.
      """
      try:
          build = ProjectBuild.objects.get(build_key=build_key)
          return reverse(
              "project_projectbuild_detail",
              kwargs={"project_pk": build.project.pk, "build_pk": build.pk})
      except ProjectBuild.DoesNotExist:
          return ""
  
subject: Change the project_tags parameter to use build_key which makes it clearer.