sbt: Dependent project does not compile first

Question

In my build.sbt (nothing fancy) ....

val common: Project =
  project
    .in(file("common"))
    .enablePlugins(...)
    .settings(libraryDependencies ++= ...)

val root =
project
      .in(file("."))
      .enablePlugins(...)
      .dependsOn(common)
      .aggregate(
        common,
        ....
      )
      .settings(...)

Problem

common does not compile before root, and so the compilation fails complaining that it cannot find those classes (in common)

FYI

• I have tried a multitude of things that I came across when searching some information on this problem (google, documentation, github issues etc.) No luck.
• sbt v1.4.9 (Play project sbt-play v2.7.9)
• The build.sbt is much bigger than what you see above (dependencies, tasks etc.). Otherwise, I don't think if there's anything special or tricky about it.

Help much appreciated!

Answer 1

I'd agree with Mario Galic on using lazy val. In fact, I'd recommend using lazy val at all times in build.sbt.

If there is a cycle, like common referring back to root, one technique you can use is to use LocalProject("project name"), like LocalProject("root"):

lazy val common = (project in file("common"))
  .settings(
    Test / test := (LocalProject("root") / Test / test).value
  )

Answer 2

To avoid initialisation problems try declaring projects as lazy vals

lazy val common = ...
lazy val root = ...

instead of strict vals

val common = ...
val root = ...

As a side note use dependsOn to establish ordering between subprojects, and not aggregate, because aggregate will not modify classpaths.