fix: subcategories are not properly handled for budget allocations (#1844)

* fix: `allocated_spending` logic

* fix: subcategories exceeding parent limit

* refactor: budget allocations and max allocation logic

* feat: add stream for budget category form validation

* feat: update uncategorized value via stream, refactor confirm button with stream

* fix: ensure live updates for parent & sibling budgets in Turbo Stream

* fix: lint issues

app/models/budget.rb

@@ -133,10 +133,10 @@ class Budget < ApplicationRecord
   end
 
   # =============================================================================
-  # Budget allocations: How much user has budgeted for all categories combined
+  # Budget allocations: How much user has budgeted for all parent categories combined
   # =============================================================================
   def allocated_spending
-    budget_categories.sum(:budgeted_spending)
+    budget_categories.reject { |bc| bc.subcategory? }.sum(&:budgeted_spending)
   end
 
   def allocated_percent

app/models/budget_category.rb

@@ -79,4 +79,29 @@ class BudgetCategory < ApplicationRecord
 
     segments
   end
+
+  def siblings
+    budget.budget_categories.select { |bc| bc.category.parent_id == category.parent_id && bc.id != id }
+  end
+
+  def max_allocation
+    return nil unless subcategory?
+
+    parent_budget = budget.budget_categories.find { |bc| bc.category.id == category.parent_id }&.budgeted_spending
+    siblings_budget = siblings.sum(&:budgeted_spending)
+
+    [ parent_budget - siblings_budget, 0 ].max
+  end
+
+  def subcategories
+    return BudgetCategory.none unless category.parent_id.nil?
+
+    budget.budget_categories
+      .joins(:category)
+      .where(categories: { parent_id: category.id })
+  end
+
+  def subcategory?
+    category.parent_id.present?
+  end
 end