How to test :dependent => :destroy using minitest::spec?

project_spec.rb

describe Project do
  
  it "should be valid" do
    project = Fabricate.build :project
    project.must_be_valid
  end
  
  it "should have a name" do
    project_without_name = Fabricate.build :project, name: nil
    project_without_name.wont_be_valid
  end
  
  it "it may have a project-number of 0" do
    project_without_number = Fabricate.build :project, number: 0
    project_without_number.must_be_valid
  end
  
  # How to test this?
  it "should destroy all associated work units when it gets deleted" do
    project_with_work_units = Fabricate.build :project
    work_unit = Fabricate.build :work_unit, project: project_with_work_units
    project_with_work_units.work_units << work_unit
    
    project_with_work_units.save!
    project_with_work_units.destroy
    
    Project.count.must_equal 0
    WorkUnit.count.must_equal 0
  end
  
end