SERVER-25139 Add tests to verify that $lookup respects the collation.

Author: visemet

jstests/aggregation/sources/lookup/collation_lookup.js

@@ -0,0 +1,136 @@
+/**
+ * Tests that the $lookup stage respects the collation.
+ *
+ * The comparison of string values between the 'localField' and 'foreignField' should use the
+ * collation either explicitly set on the aggregation operation, or the collation inherited from the
+ * collection the "aggregate" command was performed on.
+ */
+(function() {
+    "use strict";
+
+    load("jstests/aggregation/extras/utils.js");  // for arrayEq
+
+    const caseInsensitive = {collation: {locale: "en_US", strength: 2}};
+
+    const withDefaultCollationColl = db.collation_lookup_with_default;
+    const withoutDefaultCollationColl = db.collation_lookup_without_default;
+
+    withDefaultCollationColl.drop();
+    withoutDefaultCollationColl.drop();
+
+    assert.commandWorked(db.createCollection(withDefaultCollationColl.getName(), caseInsensitive));
+    assert.writeOK(withDefaultCollationColl.insert({_id: "lowercase", str: "abc"}));
+
+    assert.writeOK(withoutDefaultCollationColl.insert({_id: "lowercase", str: "abc"}));
+    assert.writeOK(withoutDefaultCollationColl.insert({_id: "uppercase", str: "ABC"}));
+    assert.writeOK(withoutDefaultCollationColl.insert({_id: "unmatched", str: "def"}));
+
+    // Test that the $lookup stage respects the inherited collation.
+    let res = withDefaultCollationColl
+                  .aggregate([{
+                      $lookup: {
+                          from: withoutDefaultCollationColl.getName(),
+                          localField: "str",
+                          foreignField: "str",
+                          as: "matched",
+                      },
+                  }])
+                  .toArray();
+    assert.eq(1, res.length, tojson(res));
+
+    let expected = [{_id: "lowercase", str: "abc"}, {_id: "uppercase", str: "ABC"}];
+    assert(
+        arrayEq(expected, res[0].matched),
+        "Expected " + tojson(expected) + " to equal " + tojson(res[0].matched) + " up to ordering");
+
+    // Test that the $lookup stage respects the inherited collation when it optimizes with an
+    // $unwind stage.
+    res = withDefaultCollationColl
+              .aggregate([
+                  {
+                    $lookup: {
+                        from: withoutDefaultCollationColl.getName(),
+                        localField: "str",
+                        foreignField: "str",
+                        as: "matched",
+                    },
+                  },
+                  {$unwind: "$matched"},
+              ])
+              .toArray();
+    assert.eq(2, res.length, tojson(res));
+
+    expected = [
+        {_id: "lowercase", str: "abc", matched: {_id: "lowercase", str: "abc"}},
+        {_id: "lowercase", str: "abc", matched: {_id: "uppercase", str: "ABC"}}
+    ];
+    assert(arrayEq(expected, res),
+           "Expected " + tojson(expected) + " to equal " + tojson(res) + " up to ordering");
+
+    // Test that the $lookup stage respects an explicit collation on the aggregation operation.
+    res = withoutDefaultCollationColl
+              .aggregate(
+                  [
+                    {$match: {_id: "lowercase"}},
+                    {
+                      $lookup: {
+                          from: withoutDefaultCollationColl.getName(),
+                          localField: "str",
+                          foreignField: "str",
+                          as: "matched",
+                      },
+                    },
+                  ],
+                  caseInsensitive)
+              .toArray();
+    assert.eq(1, res.length, tojson(res));
+
+    expected = [{_id: "lowercase", str: "abc"}, {_id: "uppercase", str: "ABC"}];
+    assert(
+        arrayEq(expected, res[0].matched),
+        "Expected " + tojson(expected) + " to equal " + tojson(res[0].matched) + " up to ordering");
+
+    // Test that the $lookup stage respects an explicit collation on the aggregation operation when
+    // it optimizes with an $unwind stage.
+    res = withoutDefaultCollationColl
+              .aggregate(
+                  [
+                    {$match: {_id: "lowercase"}},
+                    {
+                      $lookup: {
+                          from: withoutDefaultCollationColl.getName(),
+                          localField: "str",
+                          foreignField: "str",
+                          as: "matched",
+                      },
+                    },
+                    {$unwind: "$matched"},
+                  ],
+                  caseInsensitive)
+              .toArray();
+    assert.eq(2, res.length, tojson(res));
+
+    expected = [
+        {_id: "lowercase", str: "abc", matched: {_id: "lowercase", str: "abc"}},
+        {_id: "lowercase", str: "abc", matched: {_id: "uppercase", str: "ABC"}}
+    ];
+    assert(arrayEq(expected, res),
+           "Expected " + tojson(expected) + " to equal " + tojson(res) + " up to ordering");
+
+    // Test that the $lookup stage uses the "simple" collation if a collation isn't set on the
+    // collection or the aggregation operation.
+    res = withoutDefaultCollationColl
+              .aggregate([
+                  {$match: {_id: "lowercase"}},
+                  {
+                    $lookup: {
+                        from: withDefaultCollationColl.getName(),
+                        localField: "str",
+                        foreignField: "str",
+                        as: "matched",
+                    },
+                  },
+              ])
+              .toArray();
+    assert.eq([{_id: "lowercase", str: "abc", matched: [{_id: "lowercase", str: "abc"}]}], res);
+})();

src/mongo/db/pipeline/document_source.h

@@ -1664,8 +1664,6 @@ class DocumentSourceGeoNear : public DocumentSourceNeedsMongod, public Splittabl
 /**
  * Queries separate collection for equality matches with documents in the pipeline collection.
  * Adds matching documents to a new array field in the input document.
- *
- * TODO SERVER-25139: Make $lookup respect the collation.
  */
 class DocumentSourceLookUp final : public DocumentSourceNeedsMongod,
                                    public SplittableDocumentSource {