DEV: Category#with_ancestor_ids

app/models/category.rb

@@ -262,6 +262,46 @@ def self.preload_user_fields!(guardian, categories)
  end
  end
 
+ def self.with_ancestor_ids
+ # We're building up ancestor_ids by starting at the category and
+ # iteratively working our way up the chain. For performance, during the
+ # iteration, the first element is in a separate field of the query
+ categories =
+ select(
+ "categories.id",
+ "categories.parent_category_id AS ancestor_id",
+ "ARRAY[]::integer[] AS ancestor_ids",
+ )
+
+ # We already have the category and the parent_category_id, so -2
+ (SiteSetting.max_category_nesting - 2).times do
+ # Find the next ancestor and push the current ancestor onto ancestor_ids
+ categories =
+ Category.from("(#{categories.to_sql}) AS category_ancestors").select(
+ "category_ancestors.id",
+ "(SELECT c.parent_category_id FROM categories c WHERE c.id = category_ancestors.ancestor_id) AS ancestor_id",
+ "category_ancestors.ancestor_id || category_ancestors.ancestor_ids AS ancestor_ids",
+ )
+ end
+
+ # Add the last ancestor_id to ancestor_ids and remove NULLs
+ categories =
+ Category.from("(#{categories.to_sql}) AS category_ancestors").select(
+ "category_ancestors.id",
+ "(SELECT array_remove(category_ancestors.ancestor_id || category_ancestors.ancestor_ids, NULL)) AS ancestor_ids",
+ )
+
+ # Join to get the other columns from categories and use Category.from one
+ # more time so that ancestor_ids exists on the relation called
+ # 'categories'.
+ categories =
+ Category.joins(
+ "INNER JOIN (#{categories.to_sql}) AS category_ancestors ON categories.id = category_ancestors.id",
+ ).select("categories.*", "category_ancestors.ancestor_ids")
+
+ Category.from("(#{categories.to_sql}) AS categories")
+ end
+
  def self.ancestors_of(category_ids)
  ancestor_ids = []