Refactor RelativePositioning so that it can be used by other classes

app/models/concerns/relative_positioning.rb

@@ -10,12 +10,12 @@ module RelativePositioning
     after_save :save_positionable_neighbours
   end
 
-  def project_ids
-    [project.id]
+  def min_relative_position
+    self.class.in_parents(parent_ids).minimum(:relative_position)
   end
 
   def max_relative_position
-    self.class.in_projects(project_ids).maximum(:relative_position)
+    self.class.in_parents(parent_ids).maximum(:relative_position)
   end
 
   def prev_relative_position
@@ -23,7 +23,7 @@ module RelativePositioning
 
     if self.relative_position
       prev_pos = self.class
-        .in_projects(project_ids)
+        .in_parents(parent_ids)
         .where('relative_position < ?', self.relative_position)
         .maximum(:relative_position)
     end
@@ -36,7 +36,7 @@ module RelativePositioning
 
     if self.relative_position
       next_pos = self.class
-        .in_projects(project_ids)
+        .in_parents(parent_ids)
         .where('relative_position > ?', self.relative_position)
         .minimum(:relative_position)
     end
@@ -63,7 +63,7 @@ module RelativePositioning
     pos_after = before.next_relative_position
 
     if before.shift_after?
-      issue_to_move = self.class.in_projects(project_ids).find_by!(relative_position: pos_after)
+      issue_to_move = self.class.in_parents(parent_ids).find_by!(relative_position: pos_after)
       issue_to_move.move_after
       @positionable_neighbours = [issue_to_move] # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
@@ -78,7 +78,7 @@ module RelativePositioning
     pos_before = after.prev_relative_position
 
     if after.shift_before?
-      issue_to_move = self.class.in_projects(project_ids).find_by!(relative_position: pos_before)
+      issue_to_move = self.class.in_parents(parent_ids).find_by!(relative_position: pos_before)
       issue_to_move.move_before
       @positionable_neighbours = [issue_to_move] # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
@@ -92,6 +92,10 @@ module RelativePositioning
     self.relative_position = position_between(max_relative_position || START_POSITION, MAX_POSITION)
   end
 
+  def move_to_start
+    self.relative_position = position_between(min_relative_position || START_POSITION, MIN_POSITION)
+  end
+
   # Indicates if there is an issue that should be shifted to free the place
   def shift_after?
     next_pos = next_relative_position

app/models/issue.rb

@@ -35,6 +35,8 @@ class Issue < ActiveRecord::Base
 
   validates :project, presence: true
 
+  alias_attribute :parent_id, :project_id
+
   scope :in_projects, ->(project_ids) { where(project_id: project_ids) }
 
   scope :assigned, -> { where('EXISTS (SELECT TRUE FROM issue_assignees WHERE issue_id = issues.id)') }
@@ -78,6 +80,10 @@ class Issue < ActiveRecord::Base
 
   acts_as_paranoid
 
+  class << self
+    alias_method :in_parents, :in_projects
+  end
+
   def self.reference_prefix
     '#'
   end