Make RelativePositioning reusable

RelativePositioning module was heavily dependent on the Issue model.
This changes makes it easier to reuse the functionality provided by
RelativePositioning in other models.

Needed by: https://gitlab.com/gitlab-org/gitlab-ee/issues/12196

2 files changed, 39 insertions, 19 deletions

diff --git a/app/models/concerns/relative_positioning.rb b/app/models/concerns/relative_positioning.rb
index e4fe46d722a..9cd7b8d6258 100644
--- a/app/models/concerns/relative_positioning.rb
+++ b/app/models/concerns/relative_positioning.rb
@@ -1,5 +1,26 @@
 # frozen_string_literal: true
 
+# This module makes it possible to handle items as a list, where the order of items can be easily altered
+# Requirements:
+#
+# - Only works for ActiveRecord models
+# - relative_position integer field must present on the model
+# - This module uses GROUP BY: the model should have a parent relation, example: project -> issues, project is the parent relation (issues table has a parent_id column)
+#
+# Setup like this in the body of your class:
+#
+#     include RelativePositioning
+#
+#     # base query used for the position calculation
+#     def self.relative_positioning_query_base(issue)
+#       where(deleted: false)
+#     end
+#
+#     # column that should be used in GROUP BY
+#     def self.relative_positioning_parent_column
+#       :project_id
+#     end
+#
 module RelativePositioning
   extend ActiveSupport::Concern
 
@@ -93,7 +114,7 @@ module RelativePositioning
     return move_after(before) unless after
     return move_before(after) unless before
 
-    # If there is no place to insert an issue we need to create one by moving the before issue closer
+    # If there is no place to insert an item we need to create one by moving the before item closer
     # to its predecessor. This process will recursively move all the predecessors until we have a place
     if (after.relative_position - before.relative_position) < 2
       before.move_before
@@ -108,11 +129,11 @@ module RelativePositioning
     pos_after = before.next_relative_position
 
     if before.shift_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
+      item_to_move = self.class.relative_positioning_query_base(self).find_by!(relative_position: pos_after)
+      item_to_move.move_after
+      @positionable_neighbours = [item_to_move] # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
-      pos_after = issue_to_move.relative_position
+      pos_after = item_to_move.relative_position
     end
 
     self.relative_position = self.class.position_between(pos_before, pos_after)
@@ -123,11 +144,11 @@ module RelativePositioning
     pos_before = after.prev_relative_position
 
     if after.shift_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
+      item_to_move = self.class.relative_positioning_query_base(self).find_by!(relative_position: pos_before)
+      item_to_move.move_before
+      @positionable_neighbours = [item_to_move] # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
-      pos_before = issue_to_move.relative_position
+      pos_before = item_to_move.relative_position
     end
 
     self.relative_position = self.class.position_between(pos_before, pos_after)
@@ -141,13 +162,13 @@ module RelativePositioning
     self.relative_position = self.class.position_between(min_relative_position || START_POSITION, MIN_POSITION)
   end
 
-  # Indicates if there is an issue that should be shifted to free the place
+  # Indicates if there is an item that should be shifted to free the place
   def shift_after?
     next_pos = next_relative_position
     next_pos && (next_pos - relative_position) == 1
   end
 
-  # Indicates if there is an issue that should be shifted to free the place
+  # Indicates if there is an item that should be shifted to free the place
   def shift_before?
     prev_pos = prev_relative_position
     prev_pos && (relative_position - prev_pos) == 1
@@ -159,7 +180,7 @@ module RelativePositioning
   def save_positionable_neighbours
     return unless @positionable_neighbours
 
-    status = @positionable_neighbours.all? { |issue| issue.save(touch: false) }
+    status = @positionable_neighbours.all? { |item| item.save(touch: false) }
     @positionable_neighbours = nil
 
     status
@@ -170,16 +191,15 @@ module RelativePositioning
     # When calculating across projects, this is much more efficient than
     # MAX(relative_position) without the GROUP BY, due to index usage:
     # https://gitlab.com/gitlab-org/gitlab-ce/issues/54276#note_119340977
-    relation = self.class
-                 .in_parents(parent_ids)
+    relation = self.class.relative_positioning_query_base(self)
                  .order(Gitlab::Database.nulls_last_order('position', 'DESC'))
+                 .group(self.class.relative_positioning_parent_column)
                  .limit(1)
-                 .group(self.class.parent_column)
 
     relation = yield relation if block_given?
 
     relation
-      .pluck(self.class.parent_column, Arel.sql("#{calculation}(relative_position) AS position"))
+      .pluck(self.class.relative_positioning_parent_column, Arel.sql("#{calculation}(relative_position) AS position"))
       .first&.
       last
   end
diff --git a/app/models/issue.rb b/app/models/issue.rb
index 8c5dd5e382e..164858dc432 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -91,11 +91,11 @@ class Issue < ApplicationRecord
     end
   end
 
-  class << self
-    alias_method :in_parents, :in_projects
+  def self.relative_positioning_query_base(issue)
+    in_projects(issue.parent_ids)
   end
 
-  def self.parent_column
+  def self.relative_positioning_parent_column
     :project_id
   end