feat: Add isSubjectPopulated precondition

README.md

@@ -79,6 +79,20 @@ $writtenEvents = $client->writeEvents([
 ]);
 ```
 
+#### Using the `isSubjectPopulated` precondition
+
+If you only want to write events in case a subject (such as `/books/42`) already has at least one event, import the `IsSubjectPopulated` class, use it to create a precondition, and pass it in an array as the second argument:
+
+```php
+use Thenativeweb\Eventsourcingdb\IsSubjectPopulated;
+
+$writtenEvents = $client->writeEvents([
+  // events
+], [
+  new IsSubjectPopulated('/books/42'),
+]);
+```
+
 #### Using the `isSubjectOnEventId` precondition
 
 If you only want to write events in case the last event of a subject (such as `/books/42`) has a specific ID (e.g., `0`), import the `IsSubjectOnEventId` class, use it to create a precondition, and pass it in an array as the second argument:

src/IsSubjectPopulated.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Thenativeweb\Eventsourcingdb;
+
+use JsonSerializable;
+
+final readonly class IsSubjectPopulated implements JsonSerializable
+{
+    public function __construct(
+        public string $subject,
+    ) {
+    }
+
+    public function jsonSerialize(): array
+    {
+        return [
+            'type' => 'isSubjectPopulated',
+            'payload' => [
+                'subject' => $this->subject,
+            ],
+        ];
+    }
+}

tests/WriteEventsTest.php

@@ -7,6 +7,7 @@
 use Thenativeweb\Eventsourcingdb\EventCandidate;
 use Thenativeweb\Eventsourcingdb\IsEventQlQueryTrue;
 use Thenativeweb\Eventsourcingdb\IsSubjectOnEventId;
+use Thenativeweb\Eventsourcingdb\IsSubjectPopulated;
 use Thenativeweb\Eventsourcingdb\IsSubjectPristine;
 use Thenativeweb\Eventsourcingdb\Tests\Trait\ClientTestTrait;
 
@@ -104,6 +105,68 @@ public function testSupportsTheIsSubjectPristinePrecondition(): void
         );
     }
 
+    public function testRejectsWritingToEmptySubjectWhenUsingTheIsSubjectPopulatedPrecondition(): void
+    {
+        $eventCandidate = new EventCandidate(
+            source: 'https://www.eventsourcingdb.io',
+            subject: '/test',
+            type: 'io.eventsourcingdb.test',
+            data: [
+                'value' => 42,
+            ],
+        );
+
+        $this->expectExceptionMessage("Failed to write events, got HTTP status code '409', expected '200'");
+
+        $this->client->writeEvents(
+            [
+                $eventCandidate,
+            ],
+            [
+                new IsSubjectPopulated('/test'),
+            ],
+        );
+    }
+
+    public function testSupportsTheIsSubjectPopulatedPrecondition(): void
+    {
+        $firstEvent = new EventCandidate(
+            source: 'https://www.eventsourcingdb.io',
+            subject: '/test',
+            type: 'io.eventsourcingdb.test',
+            data: [
+                'value' => 23,
+            ],
+        );
+
+        $secondEvent = new EventCandidate(
+            source: 'https://www.eventsourcingdb.io',
+            subject: '/test',
+            type: 'io.eventsourcingdb.test',
+            data: [
+                'value' => 42,
+            ],
+        );
+
+        $this->client->writeEvents([
+            $firstEvent,
+        ]);
+
+        $writtenEvents = $this->client->writeEvents(
+            [
+                $secondEvent,
+            ],
+            [
+                new IsSubjectPopulated('/test'),
+            ],
+        );
+
+        $this->assertCount(1, $writtenEvents);
+        $this->assertInstanceOf(CloudEvent::class, $writtenEvents[0]);
+        $this->assertSame('1', $writtenEvents[0]->id);
+        $this->assertSame(42, $writtenEvents[0]->data['value']);
+    }
+
     public function testSupportsTheIsSubjectOnEventIdPrecondition(): void
     {
         $firstEvent = new EventCandidate(