Heroku CI with Postgres postgis extension

I'm using a postgis extension to my database on Heroku. In production this works just fine; I'm mapping like a boss.

But, I can't get my app to build on HerokuCI because the database addon in the test environment doesn't have GDAL installed (part of the postgis extension):

django.core.exceptions.ImproperlyConfigured: Could not find the GDAL library (tried "gdal", "GDAL", "gdal2.3.0", "gdal2.2.0", "gdal2.1.0", "gdal2.0.0", "gdal1.11.0"). Is GDAL installed? If it is, try setting GDAL_LIBRARY_PATH in your settings.
-----> test command `python manage.py migrate --noinput && python manage.py test` failed with exit status 1

I can't find out how to create extensions in the heroku CI environment. Heroku's app json schema say nothing about how to create extensions on the test databases.

I know that I can't use in-dyno database for CI, as postgis is not an available extension for that, but I'm not using an in-dyno one, I'm using a plan...

Here's my app schema:

{
  "name": "mymegaapp",
  "scripts": {
    "heroku-prebuild": "create extension postgis",
    "heroku-postbuild": "echo heroku-postbuild script runs here."
  },
  "env": {
  },
  "formation": {
  },
  "addons": [
  ],
  "buildpacks": [
    {
      "url": "heroku/python"
    },
    {
      "url": "heroku/nodejs"
    }
  ],
  "environments": {
    "test": {
      "scripts": {
        "test": "python manage.py migrate --noinput && python manage.py test"
      },
      "env": {
        "DJANGO_DEBUG": "1"
      },
      "addons":[
        {
          "plan": "heroku-postgresql",
          "options": {
            "version": "11"
          }
        },
        "heroku-redis:hobby-dev"
      ]
    }
  }
}

Note that I've tried to create the postgis extension in the prebuild script, which doesn't make a difference.

I've also tried invoking the creation before the test script:

      "scripts": {
        "test": "heroku create extension postgis && python manage.py migrate --noinput && python manage.py test"
      },

which doesn't work because the heroku cli isn't installed in the dyno.

The key questions:

1. How can I create extensions in HerokuCI?
2. Is postgis an allowable extension at all in HerokuCI?

You've almost got it. There's no first class support for creating pg extensions directly from the CLI. However, you can pass a statement into psql.

Try this in your postdeploy or test setup script: psql -c "create extension postgis" $DATABASE_URL As you're aware, you'll need to do this with a real database, not in-dyno. If you've attached the database as something other than DATABASE_URL, you will need to update that in the script.

[Edit by OP to add useful info] In this case, GDAL libraries will also be needed, which can be installed by setting the BUILD_WITH_GEO_LIBRARIES environment variable to 1. The final working solution in app.json will be:

  "environments": {
    "test": {
      "scripts": {
        "postdeploy": "psql -c \"create extension postgis\" $DATABASE_URL",
        "test": "python manage.py migrate --noinput && python manage.py test"
      },
      "env": {
        "DJANGO_DEBUG": "1",
        "BUILD_WITH_GEO_LIBRARIES": "1"
      },
      "addons":[
        {
          "plan": "heroku-postgresql",
          "options": {
            "version": "11"
          }
        }
      ]
    }
  }