PHPLIB-1361 Add test on $let expression operator (mongodb#55)

Author: GromNaN

generator/config/expression/let.yaml

@@ -21,3 +21,26 @@ arguments:
             - expression
         description: |
             The expression to evaluate.
+tests:
+    -
+        name: 'Example'
+        link: 'https://www.mongodb.com/docs/manual/reference/operator/aggregation/let/#example'
+        pipeline:
+            -
+                $project:
+                    finalTotal:
+                        $let:
+                            vars:
+                                total:
+                                    $add:
+                                        - '$price'
+                                        - '$tax'
+                                discounted:
+                                    $cond:
+                                        if: '$applyDiscount'
+                                        then: 0.9
+                                        else: 1
+                            in:
+                                $multiply:
+                                    - '$$total'
+                                    - '$$discounted'

tests/Builder/Expression/LetOperatorTest.php

@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+namespace MongoDB\Tests\Builder\Expression;
+
+use MongoDB\Builder\Expression;
+use MongoDB\Builder\Pipeline;
+use MongoDB\Builder\Stage;
+use MongoDB\Tests\Builder\PipelineTestCase;
+
+use function MongoDB\object;
+
+/**
+ * Test $let expression
+ */
+class LetOperatorTest extends PipelineTestCase
+{
+    public function testExample(): void
+    {
+        $pipeline = new Pipeline(
+            Stage::project(
+                finalTotal: Expression::let(
+                    vars: object(
+                        total: Expression::add(
+                            Expression::numberFieldPath('price'),
+                            Expression::numberFieldPath('tax'),
+                        ),
+                        discounted: Expression::cond(
+                            if: Expression::boolFieldPath('applyDiscount'),
+                            then: 0.9,
+                            else: 1,
+                        ),
+                    ),
+                    in: Expression::multiply(
+                        Expression::variable('total'),
+                        Expression::variable('discounted'),
+                    ),
+                ),
+            ),
+        );
+
+        $this->assertSamePipeline(Pipelines::LetExample, $pipeline);
+    }
+}