SBT generate code using project defined generator

I'd like to compile a project which contains a java source generator and then compile the generated code within a single project. I.e: compile Generator.scala, run Generator.generate(outputDir), compile outputDir, package into a jar. I'm trying this:

sourceGenerators in Compile <+= sourceManaged in Compile map { out =>
    Generator.generate(out / "generated")
}

but sbt complains

[error] Build.scala:1: object example is not a member of package org
[error] import org.example.Generator

Basically, sbt doesn't see Generator defined in the project it compiles. Is it possible to do it my way with sbt?