transfer: Support transfers of different currencies between accounts. (#2243)

Fixes part of #1852.

Co-authored-by: Zach Gollwitzer <zach@maybe.co>

app/models/family/auto_transfer_matchable.rb

@@ -9,8 +9,6 @@ module Family::AutoTransferMatchable
         JOIN entries outflow_candidates ON (
           inflow_candidates.amount < 0 AND
           outflow_candidates.amount > 0 AND
-          inflow_candidates.amount = -outflow_candidates.amount AND
-          inflow_candidates.currency = outflow_candidates.currency AND
           inflow_candidates.account_id <> outflow_candidates.account_id AND
           inflow_candidates.date BETWEEN outflow_candidates.date - 4 AND outflow_candidates.date + 4
         )
@@ -24,12 +22,26 @@ module Family::AutoTransferMatchable
         rejected_transfers.inflow_transaction_id = inflow_candidates.entryable_id AND
         rejected_transfers.outflow_transaction_id = outflow_candidates.entryable_id
       )")
+      .joins("LEFT JOIN exchange_rates ON (
+        exchange_rates.date = outflow_candidates.date AND
+        exchange_rates.from_currency = outflow_candidates.currency AND
+        exchange_rates.to_currency = inflow_candidates.currency
+      )")
       .joins("JOIN accounts inflow_accounts ON inflow_accounts.id = inflow_candidates.account_id")
       .joins("JOIN accounts outflow_accounts ON outflow_accounts.id = outflow_candidates.account_id")
       .where("inflow_accounts.family_id = ? AND outflow_accounts.family_id = ?", self.id, self.id)
       .where("inflow_accounts.is_active = true")
       .where("outflow_accounts.is_active = true")
       .where("inflow_candidates.entryable_type = 'Transaction' AND outflow_candidates.entryable_type = 'Transaction'")
+      .where("
+        (
+          inflow_candidates.currency = outflow_candidates.currency AND
+          inflow_candidates.amount = -outflow_candidates.amount
+        ) OR (
+          inflow_candidates.currency <> outflow_candidates.currency AND
+          ABS(inflow_candidates.amount / NULLIF(outflow_candidates.amount * exchange_rates.rate, 0)) BETWEEN 0.95 AND 1.05
+        )
+      ")
       .where(existing_transfers: { id: nil })
       .order("date_diff ASC") # Closest matches first
   end

test/models/family/auto_transfer_matchable_test.rb

@@ -18,6 +18,51 @@ class Family::AutoTransferMatchableTest < ActiveSupport::TestCase
     end
   end
 
+  test "auto-matches multi-currency transfers" do
+    load_exchange_prices
+    create_transaction(date: 1.day.ago.to_date, account: @depository, amount: 500)
+    create_transaction(date: Date.current, account: @credit_card, amount: -700, currency: "CAD")
+
+    assert_difference -> { Transfer.count } => 1 do
+      @family.auto_match_transfers!
+    end
+
+    # test match within lower 5% bound
+    create_transaction(date: 1.day.ago.to_date, account: @depository, amount: 1000)
+    create_transaction(date: Date.current, account: @credit_card, amount: -1330, currency: "CAD")
+
+    assert_difference -> { Transfer.count } => 1 do
+      @family.auto_match_transfers!
+    end
+
+    # test match within upper 5% bound
+    create_transaction(date: 1.day.ago.to_date, account: @depository, amount: 1500)
+    create_transaction(date: Date.current, account: @credit_card, amount: -2189, currency: "CAD")
+
+    assert_difference -> { Transfer.count } => 1 do
+      @family.auto_match_transfers!
+    end
+
+    # test no match outside of slippage tolerance
+    create_transaction(date: 1.day.ago.to_date, account: @depository, amount: 1000)
+    create_transaction(date: Date.current, account: @credit_card, amount: -1320, currency: "CAD")
+
+    assert_difference -> { Transfer.count } => 0 do
+      @family.auto_match_transfers!
+    end
+  end
+
+  test "only matches inflow with correct currency when duplicate amounts exist" do
+    load_exchange_prices
+    create_transaction(date: 1.day.ago.to_date, account: @depository, amount: 500)
+    create_transaction(date: Date.current, account: @credit_card, amount: -500, currency: "CAD")
+    create_transaction(date: Date.current, account: @credit_card, amount: -500)
+
+    assert_difference -> { Transfer.count } => 1 do
+      @family.auto_match_transfers!
+    end
+  end
+
   # In this scenario, our matching logic should find 4 potential matches.  These matches should be ranked based on
   # days apart, then de-duplicated so that we aren't auto-matching the same transaction across multiple transfers.
   test "when 2 options exist, only auto-match one at a time, ranked by days apart" do
@@ -53,4 +98,51 @@ class Family::AutoTransferMatchableTest < ActiveSupport::TestCase
       @family.auto_match_transfers!
     end
   end
+
+  test "does not match transactions outside the 4-day window" do
+    create_transaction(date: 10.days.ago.to_date, account: @depository, amount: 500)
+    create_transaction(date: Date.current, account: @credit_card, amount: -500)
+
+    assert_no_difference -> { Transfer.count } do
+      @family.auto_match_transfers!
+    end
+  end
+
+  test "does not match multi-currency transfer with missing exchange rate" do
+    create_transaction(date: Date.current, account: @depository, amount: 500)
+    create_transaction(date: Date.current, account: @credit_card, amount: -700, currency: "GBP")
+
+    assert_no_difference -> { Transfer.count } do
+      @family.auto_match_transfers!
+    end
+  end
+
+  private
+    def load_exchange_prices
+      rates = {
+        4.days.ago.to_date => 1.36,
+        3.days.ago.to_date => 1.37,
+        2.days.ago.to_date => 1.38,
+        1.day.ago.to_date  => 1.39,
+        Date.current => 1.40
+      }
+
+      rates.each do |date, rate|
+        # USD to CAD
+        ExchangeRate.create!(
+          from_currency: "USD",
+          to_currency: "CAD",
+          date: date,
+          rate: rate
+        )
+
+        # CAD to USD (inverse)
+        ExchangeRate.create!(
+          from_currency: "CAD",
+          to_currency: "USD",
+          date: date,
+          rate: (1.0 / rate).round(6)
+        )
+      end
+    end
 end