Add exchange rate feature with multi-currency transactions and transfers support (#1099)

Co-authored-by: Pedro J. Aramburu <pedro@joakin.dev>

test/models/balance/sync_cache_test.rb

@@ -0,0 +1,156 @@
+require "test_helper"
+
+class Balance::SyncCacheTest < ActiveSupport::TestCase
+  setup do
+    @family = families(:dylan_family)
+    @account = @family.accounts.create!(
+      name: "Test Account",
+      accountable: Depository.new,
+      currency: "USD",
+      balance: 1000
+    )
+  end
+
+  test "uses custom exchange rate from transaction extra field when present" do
+    # Create a transaction with EUR currency and custom exchange rate
+    _entry = @account.entries.create!(
+      date: Date.current,
+      name: "Test Transaction",
+      amount: 100,  # €100
+      currency: "EUR",
+      entryable: Transaction.new(
+        category: @family.categories.first,
+        extra: { "exchange_rate" => "1.5" }  # Custom rate: 1.5 (vs actual rate might be different)
+      )
+    )
+
+    sync_cache = Balance::SyncCache.new(@account)
+    converted_entries = sync_cache.send(:converted_entries)
+
+    converted_entry = converted_entries.first
+    assert_equal "USD", converted_entry.currency
+    assert_equal 150.0, converted_entry.amount  # 100 * 1.5 = 150
+  end
+
+  test "uses standard exchange rate lookup when custom rate not present" do
+    # Create an exchange rate in the database
+    ExchangeRate.create!(
+      from_currency: "EUR",
+      to_currency: "USD",
+      date: Date.current,
+      rate: 1.2
+    )
+
+    _entry = @account.entries.create!(
+      date: Date.current,
+      name: "Test Transaction",
+      amount: 100,  # €100
+      currency: "EUR",
+      entryable: Transaction.new(
+        category: @family.categories.first,
+        extra: {}  # No custom exchange rate
+      )
+    )
+
+    sync_cache = Balance::SyncCache.new(@account)
+    converted_entries = sync_cache.send(:converted_entries)
+
+    converted_entry = converted_entries.first
+    assert_equal "USD", converted_entry.currency
+    assert_equal 120.0, converted_entry.amount  # 100 * 1.2 = 120
+  end
+
+  test "converts multiple entries with correct rates" do
+    # Create exchange rates
+    ExchangeRate.create!(
+      from_currency: "EUR",
+      to_currency: "USD",
+      date: Date.current,
+      rate: 1.2
+    )
+    ExchangeRate.create!(
+      from_currency: "GBP",
+      to_currency: "USD",
+      date: Date.current,
+      rate: 1.27
+    )
+
+    # Create multiple entries in different currencies
+    _eur_entry = @account.entries.create!(
+      date: Date.current,
+      name: "EUR Transaction",
+      amount: 100,
+      currency: "EUR",
+      entryable: Transaction.new(
+        category: @family.categories.first,
+        extra: {}
+      )
+    )
+
+    _gbp_entry = @account.entries.create!(
+      date: Date.current,
+      name: "GBP Transaction",
+      amount: 50,
+      currency: "GBP",
+      entryable: Transaction.new(
+        category: @family.categories.first,
+        extra: {}
+      )
+    )
+
+    _usd_entry = @account.entries.create!(
+      date: Date.current,
+      name: "USD Transaction",
+      amount: 75,
+      currency: "USD",
+      entryable: Transaction.new(
+        category: @family.categories.first,
+        extra: {}
+      )
+    )
+
+    sync_cache = Balance::SyncCache.new(@account)
+    converted_entries = sync_cache.send(:converted_entries)
+
+    assert_equal 3, converted_entries.length
+
+    # All should be in USD
+    converted_entries.each { |e| assert_equal "USD", e.currency }
+
+    # Check converted amounts
+    # Sort amounts to check regardless of order
+    amounts = converted_entries.map(&:amount).sort
+    assert_in_delta 63.5, amounts[0], 0.01   # 50 GBP * 1.27
+    assert_in_delta 75.0, amounts[1], 0.01   # 75 USD * 1.0
+    assert_in_delta 120.0, amounts[2], 0.01  # 100 EUR * 1.2
+  end
+
+  test "prioritizes custom rate over fetched rate" do
+    # Create fetched rate
+    ExchangeRate.create!(
+      from_currency: "EUR",
+      to_currency: "USD",
+      date: Date.current,
+      rate: 1.2
+    )
+
+    # Create entry with custom rate that differs from fetched
+    _entry = @account.entries.create!(
+      date: Date.current,
+      name: "EUR Transaction with custom rate",
+      amount: 100,
+      currency: "EUR",
+      entryable: Transaction.new(
+        category: @family.categories.first,
+        extra: { "exchange_rate" => "1.5" }
+      )
+    )
+
+    sync_cache = Balance::SyncCache.new(@account)
+    converted_entries = sync_cache.send(:converted_entries)
+
+    converted_entry = converted_entries.first
+    # Should use custom rate (1.5), not fetched rate (1.2)
+    assert_equal 150.0, converted_entry.amount  # 100 * 1.5, not 100 * 1.2
+  end
+end