From ea7ce13a7d4b235e40134c06120eb629a9adfdf4 Mon Sep 17 00:00:00 2001
From: Michael Studman <me@michaelstudman.com>
Date: Wed, 22 Oct 2025 03:22:24 +1100
Subject: [PATCH] Increasing trades.price decimal scale (#89)

* Changing trades.price to have a larger scale - a scale of 4 causes destructive rounding when calculating transaction cost; changes to the UI to allow for inputting and showing increased scale trade prices; test case
---
 app/views/shared/_money_field.html.erb        |  6 ++--
 app/views/trades/_form.html.erb               |  2 +-
 app/views/trades/show.html.erb                |  2 ++
 ...06155348_increase_trade_price_precision.rb |  9 +++++
 db/schema.rb                                  |  2 +-
 test/models/trade_test.rb                     | 34 +++++++++++++++++++
 6 files changed, 50 insertions(+), 5 deletions(-)
 create mode 100644 db/migrate/20250806155348_increase_trade_price_precision.rb

diff --git a/app/views/shared/_money_field.html.erb b/app/views/shared/_money_field.html.erb
index 059294013..787798c44 100644
--- a/app/views/shared/_money_field.html.erb
+++ b/app/views/shared/_money_field.html.erb
@@ -48,14 +48,14 @@
           inline: true,
           placeholder: "100",
           value: if options[:value]
-            sprintf("%.#{currency.default_precision}f", options[:value])
+            sprintf("%.#{options[:precision].presence || currency.default_precision}f", options[:value])
           elsif form.object && form.object.respond_to?(amount_method)
             val = form.object.public_send(amount_method)
-            sprintf("%.#{currency.default_precision}f", val) if val.present?
+            sprintf("%.#{options[:precision].presence || currency.default_precision}f", val) if val.present?
           end,
           min: options[:min] || -99999999999999,
           max: options[:max] || 99999999999999,
-          step: currency.step,
+          step: options[:step] || currency.step,
           disabled: options[:disabled],
           data: {
             "money-field-target": "amount",
diff --git a/app/views/trades/_form.html.erb b/app/views/trades/_form.html.erb
index 57e22907a..cb248ece9 100644
--- a/app/views/trades/_form.html.erb
+++ b/app/views/trades/_form.html.erb
@@ -50,7 +50,7 @@
 
       <% if %w[buy sell].include?(type) %>
         <%= form.number_field :qty, label: t(".qty"), placeholder: "10", min: 0.000000000000000001, step: "any", required: true %>
-        <%= form.money_field :price, label: t(".price"), required: true %>
+        <%= form.money_field :price, label: t(".price"), step: 'any', precision: 10, required: true %>
       <% end %>
     </div>
 
diff --git a/app/views/trades/show.html.erb b/app/views/trades/show.html.erb
index c80b379d3..227470fba 100644
--- a/app/views/trades/show.html.erb
+++ b/app/views/trades/show.html.erb
@@ -40,6 +40,8 @@
                   disable_currency: true,
                   auto_submit: true,
                   min: 0,
+                  step: "any",
+                  precision: 10,
                   disabled: @entry.linked? %>
           <% end %>
         <% end %>
diff --git a/db/migrate/20250806155348_increase_trade_price_precision.rb b/db/migrate/20250806155348_increase_trade_price_precision.rb
new file mode 100644
index 000000000..0f9cb4020
--- /dev/null
+++ b/db/migrate/20250806155348_increase_trade_price_precision.rb
@@ -0,0 +1,9 @@
+class IncreaseTradePricePrecision < ActiveRecord::Migration[7.2]
+  def up
+    change_column :trades, :price, :decimal, precision: 19, scale: 10
+  end
+
+  def down
+    change_column :trades, :price, :decimal, precision: 19, scale: 4
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index dac00a6e5..90d190dcf 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -789,7 +789,7 @@ ActiveRecord::Schema[7.2].define(version: 2025_08_08_143007) do
   create_table "trades", id: :uuid, default: -> { "gen_random_uuid()" }, force: :cascade do |t|
     t.uuid "security_id", null: false
     t.decimal "qty", precision: 19, scale: 4
-    t.decimal "price", precision: 19, scale: 4
+    t.decimal "price", precision: 19, scale: 10
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
     t.string "currency"
diff --git a/test/models/trade_test.rb b/test/models/trade_test.rb
index c358a5ba1..c86f44210 100644
--- a/test/models/trade_test.rb
+++ b/test/models/trade_test.rb
@@ -20,4 +20,38 @@ class TradeTest < ActiveSupport::TestCase
     name = Trade.build_name("buy", 0.25, "BTC")
     assert_equal "Buy 0.25 shares of BTC", name
   end
+
+  test "price scale is preserved at 10 decimal places" do
+    security = Security.create!(ticker: "TEST", exchange_operating_mic: "XNAS")
+
+    # up to 10 decimal places — should persist exactly
+    precise_price = BigDecimal("12.3456789012")
+    trade = Trade.create!(
+      security: security,
+      price: precise_price,
+      qty: 10000,
+      currency: "USD"
+    )
+
+    trade.reload
+
+    assert_equal precise_price, trade.price
+  end
+
+  test "price is rounded to 10 decimal places" do
+    security = Security.create!(ticker: "TEST", exchange_operating_mic: "XNAS")
+
+    # over 10 decimal places — will be rounded
+    price_with_too_many_decimals = BigDecimal("1.123456789012345")
+    trade = Trade.create!(
+      security: security,
+      price: price_with_too_many_decimals,
+      qty: 1,
+      currency: "USD"
+    )
+
+    trade.reload
+
+    assert_equal BigDecimal("1.1234567890"), trade.price
+  end
 end