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

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

test/controllers/transfers_controller_test.rb

@@ -25,6 +25,143 @@ class TransfersControllerTest < ActionDispatch::IntegrationTest
     end
   end
 
+  test "can create transfer with custom exchange rate" do
+    usd_account = accounts(:depository)
+    eur_account = users(:family_admin).family.accounts.create!(
+      name: "EUR Account",
+      balance: 1000,
+      currency: "EUR",
+      accountable: Depository.new
+    )
+
+    assert_equal "USD", usd_account.currency
+    assert_equal "EUR", eur_account.currency
+
+    assert_difference "Transfer.count", 1 do
+      post transfers_url, params: {
+        transfer: {
+          from_account_id: usd_account.id,
+          to_account_id: eur_account.id,
+          date: Date.current,
+          amount: 100,
+          exchange_rate: 0.92
+        }
+      }
+    end
+
+    transfer = Transfer.where(
+      "outflow_transaction_id IN (?) AND inflow_transaction_id IN (?)",
+      usd_account.transactions.pluck(:id),
+      eur_account.transactions.pluck(:id)
+    ).last
+    assert_not_nil transfer
+    assert_equal "USD", transfer.outflow_transaction.entry.currency
+    assert_equal "EUR", transfer.inflow_transaction.entry.currency
+    assert_equal 100, transfer.outflow_transaction.entry.amount
+    assert_in_delta(-92, transfer.inflow_transaction.entry.amount, 0.01)
+  end
+
+  test "exchange_rate endpoint returns 400 when from currency is missing" do
+    get exchange_rate_url, params: {
+      to: "USD"
+    }
+
+    assert_response :bad_request
+    json_response = JSON.parse(response.body)
+    assert_equal "from and to currencies are required", json_response["error"]
+  end
+
+  test "exchange_rate endpoint returns 400 when to currency is missing" do
+    get exchange_rate_url, params: {
+      from: "EUR"
+    }
+
+    assert_response :bad_request
+    json_response = JSON.parse(response.body)
+    assert_equal "from and to currencies are required", json_response["error"]
+  end
+
+  test "exchange_rate endpoint returns 400 on invalid date format" do
+    get exchange_rate_url, params: {
+      from: "EUR",
+      to: "USD",
+      date: "not-a-date"
+    }
+
+    assert_response :bad_request
+    json_response = JSON.parse(response.body)
+    assert_equal "Invalid date format", json_response["error"]
+  end
+
+  test "exchange_rate endpoint returns rate for different currencies" do
+    ExchangeRate.expects(:find_or_fetch_rate)
+                .with(from: "USD", to: "EUR", date: Date.current)
+                .returns(OpenStruct.new(rate: 0.92))
+
+    get exchange_rate_url, params: {
+      from: "USD",
+      to: "EUR",
+      date: Date.current.to_s
+    }
+
+    assert_response :success
+    json_response = JSON.parse(response.body)
+    assert_equal 0.92, json_response["rate"]
+  end
+
+  test "exchange_rate endpoint returns error when exchange rate unavailable" do
+    ExchangeRate.expects(:find_or_fetch_rate)
+                .with(from: "USD", to: "EUR", date: Date.current)
+                .returns(nil)
+
+    get exchange_rate_url, params: {
+      from: "USD",
+      to: "EUR",
+      date: Date.current.to_s
+    }
+
+    assert_response :not_found
+    json_response = JSON.parse(response.body)
+    assert_equal "Exchange rate not found", json_response["error"]
+  end
+
+  test "cannot create transfer when exchange rate unavailable and no custom rate provided" do
+    usd_account = accounts(:depository)
+    eur_account = users(:family_admin).family.accounts.create!(
+      name: "EUR Account",
+      balance: 1000,
+      currency: "EUR",
+      accountable: Depository.new
+    )
+
+    ExchangeRate.stubs(:find_or_fetch_rate).returns(nil)
+
+    assert_no_difference "Transfer.count" do
+      post transfers_url, params: {
+        transfer: {
+          from_account_id: usd_account.id,
+          to_account_id: eur_account.id,
+          date: Date.current,
+          amount: 100
+        }
+      }
+    end
+
+    assert_response :unprocessable_entity
+  end
+
+  test "exchange_rate endpoint returns same_currency for matching currencies" do
+    get exchange_rate_url, params: {
+      from: "USD",
+      to: "USD"
+    }
+
+    assert_response :success
+    json_response = JSON.parse(response.body)
+    assert_equal true, json_response["same_currency"]
+    assert_equal 1.0, json_response["rate"]
+  end
+
   test "soft deletes transfer" do
     assert_difference -> { Transfer.count }, -1 do
       delete transfer_url(transfers(:one))