1 files changed, 56 insertions, 0 deletions
diff --git a/app/models/concerns/group_descendant.rb b/app/models/concerns/group_descendant.rb
new file mode 100644
index 00000000000..01957da0bf3
--- /dev/null
+++ b/app/models/concerns/group_descendant.rb
@@ -0,0 +1,56 @@
+module GroupDescendant
+  # Returns the hierarchy of a project or group in the from of a hash upto a
+  # given top.
+  #
+  # > project.hierarchy
+  # => { parent_group => { child_group => project } }
+  def hierarchy(hierarchy_top = nil, preloaded = nil)
+    preloaded ||= ancestors_upto(hierarchy_top)
+    expand_hierarchy_for_child(self, self, hierarchy_top, preloaded)
+  end
+
+  # Merges all hierarchies of the given groups or projects into an array of
+  # hashes. All ancestors need to be loaded into the given `descendants` to avoid
+  # queries down the line.
+  #
+  # > GroupDescendant.merge_hierarchy([project, child_group, child_group2, parent])
+  # => { parent => [{ child_group => project}, child_group2] }
+  def self.build_hierarchy(descendants, hierarchy_top = nil)
+    descendants = Array.wrap(descendants).uniq
+    return [] if descendants.empty?
+
+    unless descendants.all? { |hierarchy| hierarchy.is_a?(GroupDescendant) }
+      raise ArgumentError.new('element is not a hierarchy')
+    end
+
+    all_hierarchies = descendants.map do |descendant|
+      descendant.hierarchy(hierarchy_top, descendants)
+    end
+
+    Gitlab::Utils::MergeHash.merge(all_hierarchies)
+  end
+
+  private
+
+  def expand_hierarchy_for_child(child, hierarchy, hierarchy_top, preloaded)
+    parent = hierarchy_top if hierarchy_top && child.parent_id == hierarchy_top.id
+    parent ||= preloaded.detect { |possible_parent| possible_parent.is_a?(Group) && possible_parent.id == child.parent_id }
+
+    if parent.nil? && !child.parent_id.nil?
+      raise ArgumentError.new('parent was not preloaded')
+    end
+
+    if parent.nil? && hierarchy_top.present?
+      raise ArgumentError.new('specified top is not part of the tree')
+    end
+
+    if parent && parent != hierarchy_top
+      expand_hierarchy_for_child(parent,
+                                 { parent => hierarchy },
+                                 hierarchy_top,
+                                 preloaded)
+    else
+      hierarchy
+    end
+  end
+end