Ensure URL segment field type before using its API, and add docs around subsite and fluent domain compatibility

docs/en/introduction.md

@@ -48,3 +48,14 @@ to speak to your website administrator or hosting provider to facilitate this.
 
 
 You can simulate subsite access without setting up virtual hosts by appending ?SubsiteID=<ID> to the request.
+
+### How do Subsite domains work with Fluent domains?
+
+The Subsites module and Fluent translation module both provide the concept of defining "domains" and let you
+configure the host name for it. This functionality is essentially performing the same duty in both modules.
+
+In the "URL segment" field for CMS pages, both Subsites and Fluent will add their context to the value. If you
+have a Subsite domain configured but no Fluent domain, Fluent will respect the existing domain and add its
+locale context to the value. If you have a Subsite domain configured and a Fluent domain configured, Fluent will
+use its own domain host name value, and the Subsite domain value will be lost. For this reason, you will need
+to ensure that you use the same host name in both Subsite and Fluent domain entries.

src/Extensions/SiteTreeSubsites.php

@@ -3,6 +3,7 @@
 namespace SilverStripe\Subsites\Extensions;
 
 use Page;
+use SilverStripe\CMS\Forms\SiteTreeURLSegmentField;
 use SilverStripe\CMS\Model\SiteTree;
 use SilverStripe\Control\Controller;
 use SilverStripe\Control\Director;
@@ -150,6 +151,7 @@ public function updateCMSFields(FieldList $fields)
         // replace readonly link prefix
         $subsite = $this->owner->Subsite();
         $nested_urls_enabled = Config::inst()->get(SiteTree::class, 'nested_urls');
+        /** @var Subsite $subsite */
         if ($subsite && $subsite->exists()) {
             // Use baseurl from domain
             $baseLink = $subsite->absoluteBaseURL();
@@ -163,7 +165,7 @@ public function updateCMSFields(FieldList $fields)
             }
 
             $urlsegment = $fields->dataFieldByName('URLSegment');
-            if ($urlsegment) {
+            if ($urlsegment && $urlsegment instanceof SiteTreeURLSegmentField) {
                 $urlsegment->setURLPrefix($baseLink);
             }
         }

tests/php/SiteTreeSubsitesTest.php

@@ -5,10 +5,10 @@
 use Page;
 use SilverStripe\CMS\Controllers\CMSMain;
 use SilverStripe\CMS\Controllers\ModelAsController;
+use SilverStripe\CMS\Forms\SiteTreeURLSegmentField;
 use SilverStripe\CMS\Model\SiteTree;
 use SilverStripe\Control\Director;
 use SilverStripe\Core\Config\Config;
-use SilverStripe\Core\Convert;
 use SilverStripe\ErrorPage\ErrorPage;
 use SilverStripe\Forms\FieldList;
 use SilverStripe\Security\Member;
@@ -21,6 +21,7 @@
 use SilverStripe\Subsites\Tests\SiteTreeSubsitesTest\TestErrorPage;
 use SilverStripe\Versioned\Versioned;
 use SilverStripe\View\SSViewer;
+use TractorCow\Fluent\Extension\FluentSiteTreeExtension;
 
 class SiteTreeSubsitesTest extends BaseSubsiteTest
 {
@@ -33,7 +34,9 @@ class SiteTreeSubsitesTest extends BaseSubsiteTest
     ];
 
     protected static $illegal_extensions = [
-        SiteTree::class => ['Translatable'] // @todo implement Translatable namespace
+        SiteTree::class => [
+            FluentSiteTreeExtension::class,
+        ],
     ];
 
     protected function setUp()
@@ -449,7 +452,7 @@ public function provideAlternateAbsoluteLink()
 
     /**
      * @dataProvider provideAlternateAbsoluteLink
-     * @param name $pageFixtureName
+     * @param string $pageFixtureName
      * @param string|null $action
      * @param string $expectedAbsoluteLink
      */
@@ -465,4 +468,23 @@ public function testAlternateAbsoluteLink($pageFixtureName, $action, $expectedAb
 
         $this->assertEquals($expectedAbsoluteLink, $result);
     }
+
+    public function testURLSegmentBaseIsSetToSubsiteBaseURL()
+    {
+        // This subsite has a domain with 'one.example.org' as the primary domain
+        /** @var Subsite $subsite */
+        $subsite = $this->objFromFixture(Subsite::class, 'domaintest1');
+        Subsite::changeSubsite($subsite);
+
+        $page = new SiteTree();
+        $page->SubsiteID = $subsite->ID;
+        $page->write();
+        $fields = $page->getCMSFields();
+
+        /** @var SiteTreeURLSegmentField $urlSegmentField */
+        $urlSegmentField = $fields->dataFieldByName('URLSegment');
+        $this->assertInstanceOf(SiteTreeURLSegmentField::class, $urlSegmentField);
+
+        $this->assertSame('http://one.example.org/', $urlSegmentField->getURLPrefix());
+    }
 }