How to read environment variables in TypeSafe config in scala?

I am trying to read the environment variables in typesafe config in scala, configuring the slick database. Here is what I tried

remote_test_db = {
  dataSourceClass = "slick.jdbc.DatabaseUrlDataSource"
  properties = {
    driver = "org.postgresql.Driver"
    url = ${?REMOTE_TEST_DB_URL}
    user = ${?REMOTE_TEST_DB_USERNAME}
    password = ${?REMOTE_TEST_DB_PASSWORD}
  }
  connectionPool = disabled
  keepAliveConnection = true
}

But I get this error:

Exception in thread "main" java.lang.ExceptionInInitializerError
    at com.flyhomes.mls_pull.dump.MlsProvider.main(MlsProvider.scala)
Caused by: com.typesafe.config.ConfigException$NotResolved: need to Config#resolve(), see the API docs for Config#resolve(); substitution not resolved: ConfigConcatenation(${?REMOTE_TEST_DB_USERNAME})
    at com.typesafe.config.impl.ConfigConcatenation.notResolved(ConfigConcatenation.java:51)
    at com.typesafe.config.impl.ConfigConcatenation.valueType(ConfigConcatenation.java:58)
    at slick.util.ConfigExtensionMethods$$anonfun$slick$util$ConfigExtensionMethods$$toProps$1$1.apply(GlobalConfig.scala:71)
    at slick.util.ConfigExtensionMethods$$anonfun$slick$util$ConfigExtensionMethods$$toProps$1$1.apply(GlobalConfig.scala:69)
    at scala.collection.Iterator$class.foreach(Iterator.scala:893)
    at scala.collection.AbstractIterator.foreach(Iterator.scala:1336)
    at scala.collection.IterableLike$class.foreach(IterableLike.scala:72)
    at scala.collection.AbstractIterable.foreach(Iterable.scala:54)
    at slick.util.ConfigExtensionMethods$.slick$util$ConfigExtensionMethods$$toProps$1(GlobalConfig.scala:69)
    at slick.util.ConfigExtensionMethods$.toProperties$extension(GlobalConfig.scala:78)
    at slick.util.ConfigExtensionMethods$.getPropertiesOr$extension(GlobalConfig.scala:64)
    at slick.util.ConfigExtensionMethods$.getPropertiesOpt$extension(GlobalConfig.scala:84)
    at slick.jdbc.DataSourceJdbcDataSource$.forConfig(JdbcDataSource.scala:90)
    at slick.jdbc.DataSourceJdbcDataSource$.forConfig(JdbcDataSource.scala:86)
    at slick.jdbc.JdbcDataSource$.forConfig(JdbcDataSource.scala:48)
    at slick.jdbc.JdbcBackend$DatabaseFactoryDef$class.forConfig(JdbcBackend.scala:288)
    at slick.jdbc.JdbcBackend$$anon$3.forConfig(JdbcBackend.scala:33)
    at com.flyhomes.mls_pull.Databases$.remoteTestDb$lzycompute(Databases.scala:21)
    at com.flyhomes.mls_pull.Databases$.remoteTestDb(Databases.scala:21)
    at com.flyhomes.mls_pull.dump.MlsProvider$.<init>(MlsProvider.scala:18)
    at com.flyhomes.mls_pull.dump.MlsProvider$.<clinit>(MlsProvider.scala)
    ... 1 more

How do I access the environment variables here?

Solution

From the exception it seems that you need to call resolve - line 3: need to Config#resolve(). This will actually substitute every variable. Maybe this will help:

val config = ??? // load config here
config.resolve() // force the substitution of variables

Quoted from the docs of the class:

Resolving substitutions

Substitutions are the ${foo.bar} syntax in config files, described in the specification. Resolving substitutions replaces these references with real values.

Before using a Config it's necessary to call resolve() to handle substitutions (though ConfigFactory.load() and similar methods will do the resolve for you already).