Fixes required for dot notation support in fields

See #9163

docs/en/02_Developer_Guides/03_Forms/00_Introduction.md

@@ -332,6 +332,8 @@ class PageController extends ContentController
 
 ```
 
+See [how_tos/handle_nested_data](How to: Handle nested form data) for more advanced use cases.
+
 ## Validation
 
 Form validation is handled by the [Validator](api:SilverStripe\Forms\Validator) class and the `validator` property on the `Form` object. The validator 

src/Forms/FieldList.php

@@ -511,6 +511,7 @@ class FieldList extends ArrayList
      */
     public function fieldByName($name)
     {
+        $fullName = $name;
         if (strpos($name, '.') !== false) {
             list($name, $remainder) = explode('.', $name, 2);
         } else {
@@ -518,7 +519,9 @@ class FieldList extends ArrayList
         }
 
         foreach ($this as $child) {
-            if (trim($name) == trim($child->getName()) || $name == $child->id) {
+            if (trim($fullName) == trim($child->getName()) || $fullName == $child->id) {
+                return $child;
+            } elseif (trim($name) == trim($child->getName()) || $name == $child->id) {
                 if ($remainder) {
                     if ($child instanceof CompositeField) {
                         return $child->fieldByName($remainder);

src/Forms/Form.php

@@ -1463,10 +1463,16 @@ class Form extends ViewableData implements HasRequestHandler
 
             if (is_object($data)) {
                 // Allow dot-syntax traversal of has-one relations fields
-                if (strpos($name, '.') !== false && $data->hasMethod('relField')) {
-                    $val = $data->relField($name);
-                    $exists = true;
-
+                if (strpos($name, '.') !== false) {
+                    $exists = (
+                        $data->hasMethod('relField')
+                    );
+                    try {
+                        $val = $data->relField($name);
+                    } catch (\LogicException $e) {
+                        // There's no other way to tell whether the relation actually exists
+                        $exists = false;
+                    }
                 // Regular ViewableData access
                 } else {
                     $exists = (

tests/php/Forms/FormTest.php

@@ -88,7 +88,8 @@ class FormTest extends FunctionalTest
                 new TextField('key1'),
                 new TextField('namespace[key2]'),
                 new TextField('namespace[key3][key4]'),
-                new TextField('othernamespace[key5][key6][key7]')
+                new TextField('othernamespace[key5][key6][key7]'),
+                new TextField('dot.field')
             ),
             new FieldList()
         );
@@ -108,7 +109,9 @@ class FormTest extends FunctionalTest
                         'key7' => 'val7'
                     ]
                 ]
-            ]
+            ],
+            'dot.field' => 'dot.field val'
+
         ];
 
         $form->loadDataFrom($requestData);
@@ -118,6 +121,7 @@ class FormTest extends FunctionalTest
         $this->assertEquals('val2', $fields->fieldByName('namespace[key2]')->Value());
         $this->assertEquals('val4', $fields->fieldByName('namespace[key3][key4]')->Value());
         $this->assertEquals('val7', $fields->fieldByName('othernamespace[key5][key6][key7]')->Value());
+        $this->assertEquals('dot.field val', $fields->fieldByName('dot.field')->Value());
     }
 
     public function testSubmitReadonlyFields()
@@ -186,7 +190,8 @@ class FormTest extends FunctionalTest
                 new TextField('Name'), // appears in both Player and Team
                 new TextareaField('Biography'),
                 new DateField('Birthday'),
-                new NumericField('BirthdayYear') // dynamic property
+                new NumericField('BirthdayYear'), // dynamic property
+                new TextField('FavouriteTeam.Name') // dot syntax
             ),
             new FieldList()
         );
@@ -200,6 +205,7 @@ class FormTest extends FunctionalTest
                 'Biography' => 'Bio 1',
                 'Birthday' => '1982-01-01',
                 'BirthdayYear' => '1982',
+                'FavouriteTeam.Name' => 'Team 1',
             ],
             'LoadDataFrom() loads simple fields and dynamic getters'
         );
@@ -213,6 +219,7 @@ class FormTest extends FunctionalTest
                 'Biography' => null,
                 'Birthday' => null,
                 'BirthdayYear' => 0,
+                'FavouriteTeam.Name' => null,
             ],
             'LoadNonBlankDataFrom() loads only fields with values, and doesnt overwrite existing values'
         );
@@ -229,6 +236,7 @@ class FormTest extends FunctionalTest
                 new TextareaField('Biography'),
                 new DateField('Birthday'),
                 new NumericField('BirthdayYear'), // dynamic property
+                new TextField('FavouriteTeam.Name'), // dot syntax
                 $unrelatedField = new TextField('UnrelatedFormField')
                 //new CheckboxSetField('Teams') // relation editing
             ),
@@ -245,6 +253,7 @@ class FormTest extends FunctionalTest
                 'Biography' => 'Bio 1',
                 'Birthday' => '1982-01-01',
                 'BirthdayYear' => '1982',
+                'FavouriteTeam.Name' => 'Team 1',
                 'UnrelatedFormField' => 'random value',
             ],
             'LoadDataFrom() doesnt overwrite fields not found in the object'
@@ -261,6 +270,7 @@ class FormTest extends FunctionalTest
                 'Biography' => '',
                 'Birthday' => '',
                 'BirthdayYear' => 0,
+                'FavouriteTeam.Name' => null,
                 'UnrelatedFormField' => null,
             ],
             'LoadDataFrom() overwrites fields not found in the object with $clearMissingFields=true'