Add enum for $meta operator and add tests

generator/config/expression/meta.yaml

@@ -11,3 +11,29 @@ arguments:
         name: keyword
         type:
             - string
+tests:
+    -
+        name: 'textScore'
+        link: 'https://www.mongodb.com/docs/manual/reference/operator/aggregation/meta/#-meta---textscore-'
+        pipeline:
+            -
+                $match:
+                    $text:
+                        $search: 'cake'
+            -
+                $group:
+                    _id:
+                        $meta: 'textScore'
+                    count:
+                        $sum: 1
+    -
+        name: 'indexKey'
+        link: 'https://www.mongodb.com/docs/manual/reference/operator/aggregation/meta/#-meta---indexkey-'
+        pipeline:
+            -
+                $match:
+                    type: 'apparel'
+            -
+                $addFields:
+                    idxKey:
+                        $meta: 'indexKey'

src/Builder/Meta.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace MongoDB\Builder;
+
+use MongoDB\Builder\Type\Encode;
+use MongoDB\Builder\Type\ExpressionInterface;
+use MongoDB\Builder\Type\OperatorInterface;
+
+/**
+ * Returns the metadata associated with a document, e.g. "textScore" when performing text search.
+ *
+ * @see https://www.mongodb.com/docs/manual/reference/operator/aggregation/meta/
+ */
+enum Meta: string implements OperatorInterface, ExpressionInterface
+{
+    public const ENCODE = Encode::Single;
+
+    /**
+     * Returns the score associated with the corresponding $text query for each
+     * matching document. The text score signifies how well the document matched
+     * the search term or terms.
+     */
+    case TextScore = 'textScore';
+
+    /**
+     * Returns an index key for the document if a non-text index is used. The
+     * { $meta: "indexKey" } expression is for debugging purposes only, and
+     * not for application logic, and is preferred over cursor.returnKey().
+     */
+    case IndexKey = 'indexKey';
+
+    public function getOperator(): string
+    {
+        return '$meta';
+    }
+}

tests/Builder/Expression/MetaOperatorTest.php

@@ -0,0 +1,77 @@
+<?php
+
+declare(strict_types=1);
+
+namespace MongoDB\Tests\Builder\Expression;
+
+use MongoDB\Builder\Accumulator;
+use MongoDB\Builder\Expression;
+use MongoDB\Builder\Meta;
+use MongoDB\Builder\Pipeline;
+use MongoDB\Builder\Query;
+use MongoDB\Builder\Stage;
+use MongoDB\Tests\Builder\PipelineTestCase;
+
+/**
+ * Test $meta expression
+ */
+class MetaOperatorTest extends PipelineTestCase
+{
+    public function testIndexKey(): void
+    {
+        $pipeline = new Pipeline(
+            Stage::match(
+                type: 'apparel',
+            ),
+            Stage::addFields(
+                idxKey: Expression::meta('indexKey'),
+            ),
+        );
+
+        $this->assertSamePipeline(Pipelines::MetaIndexKey, $pipeline);
+    }
+
+    public function testIndexKeyWithEnum(): void
+    {
+        $pipeline = new Pipeline(
+            Stage::match(
+                type: 'apparel',
+            ),
+            Stage::addFields(
+                idxKey: Meta::IndexKey,
+            ),
+        );
+
+        $this->assertSamePipeline(Pipelines::MetaIndexKey, $pipeline);
+    }
+
+    public function testTextScore(): void
+    {
+        $pipeline = new Pipeline(
+            Stage::match(
+                Query::text('cake'),
+            ),
+            Stage::group(
+                _id: Expression::meta('textScore'),
+                count: Accumulator::sum(1),
+            ),
+        );
+
+        $this->assertSamePipeline(Pipelines::MetaTextScore, $pipeline);
+    }
+
+    public function testTextScoreWithEnum(): void
+    {
+        $pipeline = new Pipeline(
+            Stage::match(
+                Query::text('cake'),
+            ),
+            Stage::group(
+                _id: Meta::TextScore,
+                count: Accumulator::sum(1),
+            ),
+        );
+
+        $this->assertSamePipeline(Pipelines::MetaTextScore, $pipeline);
+    }
+}