Delete the fork network when removing the last membership

app/models/fork_network_member.rb

@@ -4,4 +4,14 @@ class ForkNetworkMember < ActiveRecord::Base
   belongs_to :forked_from_project, class_name: 'Project'
 
   validates :fork_network, :project, presence: true
+
+  after_destroy :cleanup_fork_network
+
+  private
+
+  def cleanup_fork_network
+    # Explicitly using `#count` makes sure we have the correct number if the
+    # relation was loaded in the fork_network.
+    fork_network.destroy if fork_network.fork_network_members.count == 0
+  end
 end

spec/factories/fork_network_members.rb

@@ -0,0 +1,8 @@
+FactoryGirl.define do
+  factory :fork_network_member do
+    association :project
+    association :fork_network
+
+    forked_from_project { fork_network.root_project }
+  end
+end

spec/models/fork_network_member_spec.rb

@@ -5,4 +5,22 @@ describe ForkNetworkMember do
     it { is_expected.to validate_presence_of(:project) }
     it { is_expected.to validate_presence_of(:fork_network) }
   end
+
+  describe 'destroying a ForkNetworkMember' do
+    let(:fork_network_member) { create(:fork_network_member) }
+    let(:fork_network) { fork_network_member.fork_network }
+
+    it 'removes the fork network if it was the last member' do
+      fork_network.fork_network_members.destroy_all
+
+      expect(ForkNetwork.count).to eq(0)
+    end
+
+    it 'does not destroy the fork network if there are members left' do
+      fork_network_member.destroy!
+
+      # The root of the fork network is left
+      expect(ForkNetwork.count).to eq(1)
+    end
+  end
 end