Simplify specifying finder, move ee specific code outside concern

app/controllers/concerns/issuable_collections.rb

@@ -131,9 +131,8 @@ module IssuableCollections
   end
 
   def finder
-    return @finder if @finder
+    return @finder if defined?(@finder)
 
-    @finder_type ||= finder_type
     @finder = issuable_finder_for(@finder_type)
   end
 
@@ -143,8 +142,6 @@ module IssuableCollections
                            'Issue'
                          when MergeRequestsFinder
                            'MergeRequest'
-                         when EpicsFinder
-                           'Epic'
                          end
   end
 
@@ -157,8 +154,6 @@ module IssuableCollections
                                     :source_project, :target_project, :author, :assignee, :labels, :milestone,
                                     head_pipeline: :project, target_project: :namespace, merge_request_diff: :merge_request_diff_commits
                                   ]
-                                when 'Epic'
-                                  [:group, :author]
                                 end
   end
 end

app/controllers/projects/issues_controller.rb

@@ -248,7 +248,8 @@ class Projects::IssuesController < Projects::ApplicationController
     Issues::UpdateService.new(project, current_user, update_params)
   end
 
-  def finder_type
-    IssuesFinder
+  def set_issuables_index
+    @finder_type = IssuesFinder
+    super
   end
 end

app/controllers/projects/merge_requests_controller.rb

@@ -329,7 +329,8 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     @target_branches = @merge_request.target_project.repository.branch_names
   end
 
-  def finder_type
-    MergeRequestsFinder
+  def set_issuables_index
+    @finder_type = MergeRequestsFinder
+    super
   end
 end

ee/app/controllers/groups/epics_controller.rb

@@ -53,7 +53,16 @@ class Groups::EpicsController < Groups::ApplicationController
     Epics::UpdateService.new(nil, current_user, epic_params)
   end
 
-  def finder_type
-    EpicsFinder
+  def set_issuables_index
+    @finder_type = EpicsFinder
+    super
+  end
+
+  def collection_type
+    @collection_type ||= 'Epic'
+  end
+
+  def preload_for_collection
+    @preload_for_collection ||= [:group, :author]
   end
 end