we-promise/sure · error · Category::Merger::UnauthorizedCategoryError

A parent category cannot be merged into its own subcategory

Error message

A parent category cannot be merged into its own subcategory

What it means

Category::Merger#validate_hierarchy! blocks merges where the target category is a descendant of any source category. ancestor_ids_for(target) walks the parent_id chain upward; if any ancestor id matches a source id, merging would fold a parent into its own subcategory and create a cycle in the category tree.

Source

Thrown at app/models/category/merger.rb:49

        family.transactions.where(category_id: source.id).update_all(category_id: target_category.id)
        merge_budget_categories(source)
        family.categories.where(parent_id: source.id).where.not(id: target_category.id).update_all(parent_id: target_category.id)
        family.categories.find(source.id).destroy!
        @merged_count += 1
      end
    end

    def validate_category_belongs_to_family!(category, label)
      return if category&.family_id == family.id

      raise UnauthorizedCategoryError, "#{label} does not belong to this family"
    end

    def validate_hierarchy!
      target_ancestor_ids = ancestor_ids_for(target_category)
      return unless source_categories.any? { |source| target_ancestor_ids.include?(source.id) }

      raise UnauthorizedCategoryError, "A parent category cannot be merged into its own subcategory"
    end

    def validate_reparenting!
      return if target_category.parent_id.blank?
      return unless source_categories.any? { |source| family.categories.exists?(parent_id: source.id) }

      raise UnauthorizedCategoryError, "Cannot merge a category with subcategories into a subcategory"
    end

    def ancestor_ids_for(category)
      ids = []
      seen_ids = Set.new
      current = category

      while current&.parent_id.present? && seen_ids.exclude?(current.parent_id)
        ids << current.parent_id
        seen_ids << current.parent_id
        current = family.categories.find_by(id: current.parent_id)

View on GitHub (pinned to e69894adb9)

Solutions

Pick a target outside every source's subtree (a sibling or an unrelated top-level category)
Invert the merge direction: merge the subcategory into the parent instead of the parent into the subcategory
Exclude descendants of any selected source from target candidates in the picker or a pre-merge validation layer

Defensive patterns

Strategy: validation

Validate before calling

source_ids = source_categories.map(&:id).to_set
# walk target's ancestors; reject any target inside a source subtree
ancestor = target_category
while (pid = ancestor.parent_id)
  break if source_ids.include?(pid)
  ancestor = family.categories.find_by(id: pid) or break
end
# if the loop matched, the merger will raise

Prevention

Filter target candidates to exclude descendants of any selected source
Prefer merging children into parents (direction that cannot cycle)
Cover both invalid directions (parent-into-child, child-with-children-into-child) in merge tests

When it happens

Trigger: Selecting a subcategory as the merge target while one of its ancestors is among source_categories — for example target 'Food > Groceries' with source 'Food'. Also hit when the target picker lists all family categories without excluding descendants of the selected sources.

Common situations: Category cleanup UIs that allow any category as target; bulk merges built from name matching that pair parents with their children; imports that auto-merge similarly named categories.

Related errors

AI-assisted analysis of we-promise/sure@e69894adb9 (2026-08-21). Data as JSON: /api/errors/453c0964418f1be3. Report an issue: GitHub.