Fix budget navigation to allow selecting previous months

- Allow going back 2 years minimum even without entries
- Update oldest_valid_budget_date to use min of entry date or 2 years ago
- Add comprehensive tests for budget date validation
- Fixes issue where users couldn't select prior budget months

Author: zachgoll

app/models/budget.rb

@@ -49,7 +49,10 @@ def find_or_bootstrap(family, start_date:)
 
     private
       def oldest_valid_budget_date(family)
-        @oldest_valid_budget_date ||= family.oldest_entry_date.beginning_of_month
+        # Allow going back to either the earliest entry date OR 2 years ago, whichever is earlier
+        two_years_ago = 2.years.ago.beginning_of_month
+        oldest_entry_date = family.oldest_entry_date.beginning_of_month
+        [ two_years_ago, oldest_entry_date ].min
       end
   end
 

test/models/budget_test.rb

@@ -0,0 +1,88 @@
+require "test_helper"
+
+class BudgetTest < ActiveSupport::TestCase
+  setup do
+    @family = families(:empty)
+  end
+
+  test "budget_date_valid? allows going back 2 years even without entries" do
+    two_years_ago = 2.years.ago.beginning_of_month
+    assert Budget.budget_date_valid?(two_years_ago, family: @family)
+  end
+
+  test "budget_date_valid? allows going back to earliest entry date if more than 2 years ago" do
+    # Create an entry 3 years ago
+    old_account = Account.create!(
+      family: @family,
+      accountable: Depository.new,
+      name: "Old Account",
+      status: "active",
+      currency: "USD",
+      balance: 1000
+    )
+
+    old_entry = Entry.create!(
+      account: old_account,
+      entryable: Transaction.new(category: categories(:income)),
+      date: 3.years.ago,
+      name: "Old Transaction",
+      amount: 100,
+      currency: "USD"
+    )
+
+    # Should allow going back to the old entry date
+    assert Budget.budget_date_valid?(3.years.ago.beginning_of_month, family: @family)
+  end
+
+  test "budget_date_valid? does not allow dates before earliest entry or 2 years ago" do
+    # Create an entry 1 year ago
+    account = Account.create!(
+      family: @family,
+      accountable: Depository.new,
+      name: "Test Account",
+      status: "active",
+      currency: "USD",
+      balance: 500
+    )
+
+    Entry.create!(
+      account: account,
+      entryable: Transaction.new(category: categories(:income)),
+      date: 1.year.ago,
+      name: "Recent Transaction",
+      amount: 100,
+      currency: "USD"
+    )
+
+    # Should not allow going back more than 2 years
+    refute Budget.budget_date_valid?(3.years.ago.beginning_of_month, family: @family)
+  end
+
+  test "budget_date_valid? does not allow future dates beyond current month" do
+    refute Budget.budget_date_valid?(2.months.from_now, family: @family)
+  end
+
+  test "previous_budget_param returns nil when date is too old" do
+    # Create a budget at the oldest allowed date
+    two_years_ago = 2.years.ago.beginning_of_month
+    budget = Budget.create!(
+      family: @family,
+      start_date: two_years_ago,
+      end_date: two_years_ago.end_of_month,
+      currency: "USD"
+    )
+
+    assert_nil budget.previous_budget_param
+  end
+
+  test "previous_budget_param returns param when date is valid" do
+    budget = Budget.create!(
+      family: @family,
+      start_date: Date.current.beginning_of_month,
+      end_date: Date.current.end_of_month,
+      currency: "USD"
+    )
+
+    assert_not_nil budget.previous_budget_param
+  end
+end