ENH Allow better subclassing of MoneyField

Move generation of NumberField from constructor to method to allow override in subclass.
Addded test for MoneyField

src/Forms/MoneyField.php

@@ -59,11 +59,7 @@ public function getAmountField()
     public function __construct($name, $title = null, $value = "")
     {
         $this->setName($name);
-        $this->fieldAmount = NumericField::create(
-            "{$name}[Amount]",
-            _t('SilverStripe\\Forms\\MoneyField.FIELDLABELAMOUNT', 'Amount')
-        )
-            ->setScale(2);
+        $this->buildAmountField();
         $this->buildCurrencyField();
 
         parent::__construct($name, $title, $value);
@@ -75,6 +71,20 @@ public function __clone()
         $this->fieldCurrency = clone $this->fieldCurrency;
     }
 
+    /**
+     * Builds a field to input the amount of money
+     *
+     * @return void
+     */
+    protected function buildAmountField(): void
+    {
+        $this->fieldAmount = NumericField::create(
+            $this->name . '[Amount]',
+            _t('SilverStripe\\Forms\\MoneyField.FIELDLABELAMOUNT', 'Amount')
+        )
+            ->setScale(2);
+    }
+
     /**
      * Builds a new currency field based on the allowed currencies configured
      *

tests/php/Forms/MoneyFieldTest.php

@@ -2,9 +2,13 @@
 
 namespace SilverStripe\Forms\Tests;
 
+use SilverStripe\Forms\DropdownField;
+use SilverStripe\Forms\HiddenField;
+use SilverStripe\Forms\NumericField;
 use SilverStripe\Forms\RequiredFields;
 use SilverStripe\Forms\Tests\MoneyFieldTest\CustomSetter_Object;
 use SilverStripe\Forms\Tests\MoneyFieldTest\TestObject;
+use SilverStripe\Forms\TextField;
 use SilverStripe\ORM\FieldType\DBMoney;
 use SilverStripe\Dev\SapphireTest;
 use SilverStripe\Forms\MoneyField;
@@ -127,4 +131,31 @@ public function testValidation()
         ]);
         $this->assertFalse($field->validate($validator));
     }
+
+    public function testCurrencyField(): void
+    {
+        $field = new MoneyField('Money');
+        $field->setAllowedCurrencies(['NZD', 'USD']);
+
+        $this->assertInstanceOf(DropdownField::class, $field->getCurrencyField());
+        $this->assertEquals('Money[Currency]', $field->getCurrencyField()->getName());
+
+        $field->setAllowedCurrencies(['USD']);
+
+        $this->assertInstanceOf(HiddenField::class, $field->getCurrencyField());
+        $this->assertEquals('Money[Currency]', $field->getCurrencyField()->getName());
+
+        $field->setAllowedCurrencies([]);
+
+        $this->assertInstanceOf(TextField::class, $field->getCurrencyField());
+        $this->assertEquals('Money[Currency]', $field->getCurrencyField()->getName());
+    }
+
+    public function testAmountField(): void
+    {
+        $field = new MoneyField('Money');
+        $this->assertInstanceOf(NumericField::class, $field->getAmountField());
+        $this->assertEquals(2, $field->getAmountField()->getScale());
+        $this->assertEquals('Money[Amount]', $field->getAmountField()->getName());
+    }
 }