Fixes.

src/Extension/FluentExtension.php

@@ -85,6 +85,20 @@ class FluentExtension extends DataExtension
      */
     const INHERITANCE_MODE_ANY = 'any';
 
+    /**
+     * Fluent inheritance mode for frontend
+     *
+     * @var string
+     */
+    private static string $frontend_publish_required = self::INHERITANCE_MODE_ANY;
+
+    /**
+     * Fluent inheritance mode for CMS
+     *
+     * @var string
+     */
+    private static string $cms_localisation_required = self::INHERITANCE_MODE_ANY;
+
     /**
      * DB fields to be used added in when creating a localised version of the owner's table
      *
@@ -975,7 +989,7 @@ public function getSourceLocale(): ?Locale
         $owner = $this->owner;
         $currentLocale = FluentState::singleton()->getLocale();
 
-        // We do not have a locale set, so we can't determine a source
+        // We do not have a locale set, so we can't determine the source locale
         if (!$currentLocale) {
             return null;
         }

src/Model/RecordLocale.php

@@ -4,8 +4,10 @@
 
 use SilverStripe\Control\Director;
 use SilverStripe\i18n\i18n;
+use SilverStripe\ORM\ArrayList;
 use SilverStripe\ORM\DataObject;
 use SilverStripe\Versioned\Versioned;
+use SilverStripe\View\ArrayData;
 use SilverStripe\View\ViewableData;
 use TractorCow\Fluent\Extension\FluentExtension;
 use TractorCow\Fluent\Extension\FluentFilteredExtension;
@@ -367,19 +369,80 @@ public function getSourceLocale(): ?Locale
     {
         /** @var DataObject|FluentExtension $record */
         $record = $this->getOriginalRecord();
+        $config = $record->config();
 
+        $inheritanceMode = FluentState::singleton()->getIsFrontend()
+            ? $config->get('frontend_publish_required')
+            : $config->get('cms_localisation_required');
+
+        // This model has localised data in the current locale so the current locale is also the source locale
         if ($record->existsInLocale($this->getLocale())) {
             return $this->getLocaleObject();
         }
 
+        // This model requires localisation so fallback of any kind is not allowed
+        // hence the content can't come from another locale
+        // We don't have a source locale for this case
+        if ($inheritanceMode === FluentExtension::INHERITANCE_MODE_EXACT) {
+            return null;
+        }
+
+        $fallbackLocales = [];
+
         foreach ($this->getLocaleObject()->Fallbacks() as $fallback) {
-            if (!$record->existsInLocale($fallback->Locale)) {
+            $fallbackLocale = $fallback->Locale;
+            $fallbackLocales[] = $fallbackLocale;
+
+            if (!$record->existsInLocale($fallbackLocale)) {
                 continue;
             }
 
+            // We found a locale to fall back to, so this will be our source locale
             return $fallback;
         }
 
+        // This model allows fallback to any locale even bypassing the fallback rules,
+        // so we just need to find the most recently updated locale
+        // as that's the one which is going to be providing content
+        if ($inheritanceMode === FluentExtension::INHERITANCE_MODE_ANY) {
+            $lastEditedList = ArrayList::create();
+
+            /** @var RecordLocale $localeInformation */
+            foreach ($record->Locales() as $localeInformation) {
+                $locale = $localeInformation->getLocale();
+
+                // Ignore any fallback locales as we've already confirmed that none of these locales
+                // are acting as the source locale
+                if (in_array($locale, $fallbackLocales)) {
+                    continue;
+                }
+
+                $localisedRecord = $localeInformation->getRecord();
+
+                // We don't have any localised data in this locale so this doesn't qualify as a potential source locale
+                if (!$localisedRecord) {
+                    continue;
+                }
+
+                $data = ArrayData::create([
+                    'Locale' => $locale,
+                    'LastEdited' => $localisedRecord->LastEdited,
+                ]);
+
+                $lastEditedList->push($data);
+            }
+
+            // We found at leadt one candidate for the source locale, let's pick the most recently updated one
+            if ($lastEditedList->count() > 0) {
+                $sortedLocales = $lastEditedList
+                    ->sort('LastEdited', 'DESC')
+                    ->column('Locale');
+                $mostRecentlyEditedLocale = array_shift($sortedLocales);
+
+                return Locale::getByLocale($mostRecentlyEditedLocale);
+            }
+        }
+
         return null;
     }
 }