$lookup "from" must be null when using $documents pipeline (#1218)

JAVA-5190

Author: katcharov

driver-core/src/main/com/mongodb/client/model/Aggregates.java

@@ -310,7 +310,7 @@ public static Bson lookup(final String from, final String localField, final Stri
      * the {@code from} collection is ignored.
      *
      * @param from          the name of the collection in the same database to
-     *                      perform the join with. May be {$code null} if the
+     *                      perform the join with. Must be {$code null} if the
      *                      first pipeline stage is $documents.
      * @param pipeline      the pipeline to run on the joined collection.
      * @param as            the name of the new array field to add to the input documents.
@@ -332,7 +332,7 @@ public static Bson lookup(@Nullable final String from, final List<? extends Bson
      *
      * @param <TExpression> the Variable value expression type
      * @param from          the name of the collection in the same database to
-     *                      perform the join with. May be {$code null} if the
+     *                      perform the join with. Must be {$code null} if the
      *                      first pipeline stage is $documents.
      * @param let           the variables to use in the pipeline field stages.
      * @param pipeline      the pipeline to run on the joined collection.

driver-core/src/test/functional/com/mongodb/client/model/AggregatesTest.java

@@ -275,21 +275,12 @@ public void testDocumentsLookup() {
         getCollectionHelper().insertDocuments("[{_id: 1, a: 8}, {_id: 2, a: 9}]");
         Bson documentsStage = Aggregates.documents(asList(Document.parse("{a: 5}")));
 
-        Bson lookupStage = Aggregates.lookup("ignored", Arrays.asList(documentsStage), "added");
+        Bson lookupStage = Aggregates.lookup(null, Arrays.asList(documentsStage), "added");
         assertPipeline(
-                "{'$lookup': {'from': 'ignored', 'pipeline': [{'$documents': [{'a': 5}]}], 'as': 'added'}}",
+                "{'$lookup': {'pipeline': [{'$documents': [{'a': 5}]}], 'as': 'added'}}",
                 lookupStage);
         assertEquals(
                 parseToList("[{_id:1, a:8, added: [{a: 5}]}, {_id:2, a:9, added: [{a: 5}]}]"),
                 getCollectionHelper().aggregate(Arrays.asList(lookupStage)));
-
-        // null variant
-        Bson lookupStageNull = Aggregates.lookup(null, Arrays.asList(documentsStage), "added");
-        assertPipeline(
-                "{'$lookup': {'pipeline': [{'$documents': [{'a': 5}]}], 'as': 'added'}}",
-                lookupStageNull);
-        assertEquals(
-                parseToList("[{_id:1, a:8, added: [{a: 5}]}, {_id:2, a:9, added: [{a: 5}]}]"),
-                getCollectionHelper().aggregate(Arrays.asList(lookupStageNull)));
     }
 }