Moving state_count to Milestone model

By moving and improving state_count the functions in GlobalMilestone
are no longer used.

app/models/global_milestone.rb

@@ -34,66 +34,6 @@ class GlobalMilestone
     new(title, child_milestones)
   end
-  def self.states_count(projects, groups = nil)
-    legacy_group_milestones_count = legacy_group_milestone_states_count(projects)
-    group_milestones_count = groups_milestone_state_count(groups)
-
-    legacy_group_milestones_count.merge(group_milestones_count) do |k, legacy_group_milestones_count, group_milestones_count|
-      legacy_group_milestones_count + group_milestones_count
-    end
-  end
-
-  def self.groups_milestone_state_count(groups)
-    return STATE_COUNT_HASH unless groups
-    return self.group_milestones_states_count(groups) unless groups.respond_to?(:each)
-
-    milestone_states = STATE_COUNT_HASH
-
-    groups.each do |group|
-      group_milestones_count = self.group_milestones_states_count(group)
-      milestone_states = milestone_states.merge(group_milestones_count) do |k, milestone_state, group_milestones_count|
-        milestone_state + group_milestones_count
-      end
-    end
-
-    milestone_states
-  end
-
-  def self.group_milestones_states_count(group)
-    return STATE_COUNT_HASH unless group
-
-    params = { group_ids: [group.id], state: 'all' }
-
-    relation = MilestonesFinder.new(params).execute # rubocop: disable CodeReuse/Finder
-    grouped_by_state = relation.reorder(nil).group(:state).count
-
-    {
-      opened: grouped_by_state['active'] || 0,
-      closed: grouped_by_state['closed'] || 0,
-      all: grouped_by_state.values.sum
-    }
-  end
-
-  # Counts the legacy group milestones which must be grouped by title
-  def self.legacy_group_milestone_states_count(projects)
-    return STATE_COUNT_HASH unless projects
-
-    params = { project_ids: projects.map(&:id), state: 'all' }
-
-    relation = MilestonesFinder.new(params).execute # rubocop: disable CodeReuse/Finder
-    project_milestones_by_state_and_title = relation.reorder(nil).group(:state, :title).count
-
-    opened = count_by_state(project_milestones_by_state_and_title, 'active')
-    closed = count_by_state(project_milestones_by_state_and_title, 'closed')
-    all = project_milestones_by_state_and_title.map { |(_, title), _| title }.uniq.count
-
-    {
-      opened: opened,
-      closed: closed,
-      all: all
-    }
-  end
-
   def self.count_by_state(milestones_by_state_and_title, state)
     milestones_by_state_and_title.count do |(milestone_state, _), _|
       milestone_state == state

app/models/milestone.rb

@@ -170,6 +170,22 @@ class Milestone < ActiveRecord::Base
     sorted.with_order_id_desc
   end
+  def self.states_count(projects, groups = nil)
+    return STATE_COUNT_HASH unless projects || groups
+
+    counts = Milestone
+               .for_projects_and_groups(projects&.map(&:id), groups&.map(&:id))
+               .reorder(nil)
+               .group(:state)
+               .count
+
+    {
+        opened: counts['active'] || 0,
+        closed: counts['closed'] || 0,
+        all: counts.values.sum
+    }
+  end
+
   ##
   # Returns the String necessary to reference this Milestone in Markdown. Group
   # milestones only support name references, and do not support cross-project