Include epics from subgroups

When listing epics in a group, include also epics from all group's
subgroups. Also `by_iids` filter is removed from EpicsFinder because
filtering by iid (which is scoped to single group) doesn't make sense
since subgroups are included too.

Close #4301

changelogs/unreleased-ee/ee-subepics.yml

+---
+title: Include epics from subgroups on Epic index page
+merge_request:
+author:
+type: fixed

doc/api/epics.md

@@ -12,7 +12,7 @@ The [epic issues API](epic_issues.md) allows you to interact with issues associa
 
 ## List epics for a group
 
-Gets all epics of the requested group.
+Gets all epics of the requested group and its subgroups.
 
 ```
 GET /groups/:id/-/epics

doc/user/group/epics/index.md

@@ -9,7 +9,8 @@ milestones.
 ## Creating an epic
 
 A paginated list of epics is available in each group from where you can create
-a new epic. From your group page:
+a new epic. The list of epics includes also epics from all subgroups of the
+selected group. From your group page:
 
 1. Go to **Epics**
 1. Click the **New epic** button at the top right

ee/app/finders/epics_finder.rb

@@ -10,7 +10,6 @@ class EpicsFinder < IssuableFinder
     items = by_created_at(items)
     items = by_search(items)
     items = by_author(items)
-    items = by_iids(items)
 
     sort(items)
   end
@@ -37,6 +36,16 @@ class EpicsFinder < IssuableFinder
   end
 
   def init_collection
-    group.epics
+    groups = groups_user_can_read_epics(group.self_and_descendants)
+
+    Epic.where(group: groups)
+  end
+
+  private
+
+  def groups_user_can_read_epics(groups)
+    DeclarativePolicy.user_scope do
+      groups.select { |g| Ability.allowed?(current_user, :read_epic, g) }
+    end
   end
 end

ee/app/views/groups/epics/_epic.html.haml

@@ -7,8 +7,7 @@
             = link_to epic.title, epic_path(epic)
         .issuable-info
           %span.issuable-reference
-            -# TODO: Use to_reference
-            = "&#{epic.iid}"
+            = epic.to_reference(@group)
           %span.issuable-authored.hidden-xs
             ·
             opened #{time_ago_with_tooltip(epic.created_at, placement: 'bottom')}

ee/lib/api/epics.rb

@@ -40,7 +40,7 @@ module API
         success Entities::Epic
       end
       get ':id/-/epics' do
-        present user_group.epics, with: Entities::Epic
+        present EpicsFinder.new(current_user, group_id: user_group.id).execute, with: Entities::Epic
       end
 
       desc 'Get details of an epic' do

spec/ee/spec/finders/epics_finder_spec.rb

@@ -44,7 +44,7 @@ describe EpicsFinder do
         end
       end
 
-      context 'wtih correct params' do
+      context 'with correct params' do
         before do
           group.add_developer(search_user)
         end
@@ -79,9 +79,14 @@ describe EpicsFinder do
           end
         end
 
-        context 'by iids' do
-          it 'returns all epics by the given iids' do
-            expect(epics(iids: [epic1.iid, epic3.iid])).to contain_exactly(epic1, epic3)
+        context 'when subgroups are supported', :nested_groups do
+          let(:subgroup) { create(:group, :private, parent: group) }
+          let(:subgroup2) { create(:group, :private, parent: subgroup) }
+          let!(:subepic1) { create(:epic, group: subgroup) }
+          let!(:subepic2) { create(:epic, group: subgroup2) }
+
+          it 'returns all epics that belong to the given group and its subgroups' do
+            expect(epics).to contain_exactly(epic1, epic2, epic3, subepic1, subepic2)
           end
         end
       end