How to use SBT's externalPom() command - scala

Question

I have a Maven POM file that the deployment engineers need to deploy the system in the enterprise. I have developers using SBT for a Scala project. They use SBT targets that just aren't supported in Maven. We'd like to use the Maven POM file to define the dependencies, slurp in those dependencies in SBT, and define SBT development targets there.
According to the SBT documentation, the externalPom() command is the way to do that. But even with the simplest POM file (two developers have tried this with two different simple POM files that defined different dependencies), the externalPom() command seems to half work. The SBT targets clearly recognize the dependency defined in the POM, but can't resolve it. This error arises:
Cannot add dependency 'commons-collections#commons-collections;3.2.2'
to configuration 'default' of module
default#maven-sbt$sources_javadoc;0.1-SNAPSHOT because this
configuration doesn't exist!
When the externalPom() command is commented out and the equivalent dependency added directly in the build.sbt file everything goes swimmingly. The dependency comes directly from Maven Central in both cases; one from copying the dependency from the Maven tab and one from copying the dependency from the SBT tab. Once again, two developers are seeing exactly the same thing, from two different dependencies. The thing that's common is that both developers have reduced the build.sbt file down to a single statement. In the "slurp from POM" case, that statement is externalPom(). In the "plain old SBT" case, that statement is the dependency copied from Maven Central. The POM file is a dependency list with a single dependency (as simple as we can make it and still test externalPom()).
We suspect that we need something else in the build.sbt to make the externalPom() command work but we don't know what it is. Any help with that would be greatly appreciated.

Answer 1

I did some experimentation with this, and was able to replicate your error in my experiments.
I'm still a bit of a Scala / SBT newbie, but I created a build.sbt file that looks like so:
val Default = config("default")
lazy val root = (project in file(".")).
configs(Default).
settings(
externalPom()
)
This did compile for me!
One non-obvious catch: I had to make sure to include the scala-library in my POM file as a dependency