Nullify pool_repository when a project leaves

Previously the nullification wasn't done, as the only caller would later
destroy the model all together. In
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9070 this change
was made. This commit is basically a backport before that MR is merged.

app/models/pool_repository.rb

@@ -85,7 +85,11 @@ class PoolRepository < ActiveRecord::Base
   def unlink_repository(repository)
     object_pool.unlink_repository(repository.raw)
 
-    mark_obsolete unless member_projects.where.not(id: repository.project.id).exists?
+    if member_projects.where.not(id: repository.project.id).exists?
+      true
+    else
+      mark_obsolete
+    end
   end
 
   def object_pool

app/models/project.rb

@@ -2040,7 +2040,7 @@ class Project < ActiveRecord::Base
   end
 
   def leave_pool_repository
-    pool_repository&.unlink_repository(repository)
+    pool_repository&.unlink_repository(repository) && update_column(:pool_repository_id, nil)
   end
 
   private

spec/models/project_spec.rb

@@ -4387,6 +4387,17 @@ describe Project do
     end
   end
 
+  describe '#leave_pool_repository' do
+    let(:pool) { create(:pool_repository) }
+    let(:project) { create(:project, :repository, pool_repository: pool) }
+
+    it 'removes the membership' do
+      project.leave_pool_repository
+
+      expect(pool.member_projects.reload).not_to include(project)
+    end
+  end
+
   def rugged_config
     rugged_repo(project.repository).config
   end