Add Validation to Order Creation

app/models/order.rb

@@ -54,6 +54,28 @@ def existing_transaction_type
     order_transaction_type == :debit ? "buy" : "sell"
   end
 
+  def sufficient_funds?
+    return true unless buy?
+    return true unless number_of_shares_valid_for_calculations?
+
+    current_balance_cents = (user.portfolio&.cash_balance || 0) * 100
+
+    fee_already_subtracted = user.orders.pending.exists? ? PortfolioTransaction::TRANSACTION_FEE_CENTS : 0
+    balance_without_reservations = current_balance_cents + purchase_cost + fee_already_subtracted
+
+    total_needed = purchase_cost + transaction_fee
+
+    balance_without_reservations >= total_needed
+  end
+
+  def sufficient_shares?
+    return true unless sell?
+    return true unless number_of_shares_valid_for_calculations?
+
+    current_shares = user.portfolio&.shares_owned(stock_id) || 0
+    shares <= current_shares
+  end
+
   private
 
   def number_of_shares_valid_for_calculations?

app/services/execute_order.rb

@@ -15,6 +15,11 @@ def self.execute(...)
   def execute
     return unless order.pending?
 
+    unless valid_order?
+      cancel_order!
+      return
+    end
+
     ActiveRecord::Base.transaction do
       create_portfolio_transaction
       create_portfolio_stock
@@ -54,4 +59,18 @@ def update_order_status
   def purchase_cost
     order.purchase_cost
   end
+
+  def valid_order?
+    if order.buy?
+      order.sufficient_funds?
+    elsif order.sell?
+      order.sufficient_shares?
+    end
+
+    true
+  end
+
+  def cancel_order!
+    order.cancel!
+  end
 end

test/services/execute_order_test.rb

@@ -146,4 +146,32 @@ class ExecuteOrderTest < ActiveSupport::TestCase
     total_shares = portfolio.shares_owned(stock.id)
     assert_equal 6, total_shares
   end
+
+  test "cannot create buy order with insufficient funds" do
+    portfolio = create(:portfolio)
+    portfolio.portfolio_transactions.create!(amount_cents: 500, transaction_type: :deposit)
+    stock = create(:stock, price_cents: 100)
+
+    order = build(:order, :pending, :buy, shares: 10, stock: stock, user: portfolio.user)
+
+    assert_raises(ActiveRecord::RecordInvalid) do
+      order.save!
+    end
+
+    assert order.errors[:shares].first.include?("Insufficient funds")
+  end
+
+  test "cannot create sell order with insufficient shares" do
+    portfolio = create(:portfolio)
+    stock = create(:stock, price_cents: 100)
+    create(:portfolio_stock, portfolio: portfolio, stock: stock, shares: 5, purchase_price: 100)
+
+    order = build(:order, :pending, :sell, shares: 10, stock: stock, user: portfolio.user)
+
+    assert_raises(ActiveRecord::RecordInvalid) do
+      order.save!
+    end
+
+    assert order.errors[:base].first.include?("Cannot sell more shares than you own")
+  end
 end