Teach Project about #all_clusters

This returns a union of the project level clusters and group level
clusters associated with this project.

app/models/clusters/cluster.rb

@@ -4,6 +4,7 @@ module Clusters
   class Cluster < ActiveRecord::Base
     include Presentable
     include Gitlab::Utils::StrongMemoize
+    include FromUnion
 
     self.table_name = 'clusters'
 

app/models/project.rb

@@ -1061,6 +1061,12 @@ class Project < ActiveRecord::Base
     path
   end
 
+  def all_clusters
+    group_clusters = Clusters::Cluster.joins(:groups).where(cluster_groups: { group_id: ancestors_upto } )
+
+    Clusters::Cluster.from_union([clusters, group_clusters])
+  end
+
   def items_for(entity)
     case entity
     when 'issue' then

spec/models/project_spec.rb

@@ -3984,6 +3984,27 @@ describe Project do
     end
   end
 
+  describe '#all_clusters' do
+    let(:project) { create(:project) }
+    let(:cluster) { create(:cluster, cluster_type: :project_type, projects: [project]) }
+
+    subject { project.all_clusters }
+
+    it 'returns project level cluster' do
+      expect(subject).to eq([cluster])
+    end
+
+    context 'project belongs to a group' do
+      let(:group_cluster) { create(:cluster, :group) }
+      let(:group) { group_cluster.group }
+      let(:project) { create(:project, group: group) }
+
+      it 'returns clusters for groups of this project' do
+        expect(subject).to contain_exactly(cluster, group_cluster)
+      end
+    end
+  end
+
   def rugged_config
     rugged_repo(project.repository).config
   end