Improve error handling of invalid $project specifications

Author: bwaldvogel

core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/Aggregation.java

@@ -107,7 +107,7 @@ public static Aggregation fromPipeline(List<Document> pipeline, DatabaseResolver
                     aggregation.addStage(new SortStage(orderBy));
                     break;
                 case "$project":
-                    Document projection = (Document) stage.get(stageOperation);
+                    Object projection = stage.get(stageOperation);
                     aggregation.addStage(new ProjectStage(projection));
                     break;
                 case "$count":

core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/stage/ProjectStage.java

@@ -22,12 +22,16 @@ public class ProjectStage implements AggregationStage {
     private final Document projection;
     private final boolean hasInclusions;
 
-    public ProjectStage(Document projection) {
-        if (projection.isEmpty()) {
+    public ProjectStage(Object projection) {
+        if (!(projection instanceof Document)) {
+            throw new MongoServerError(15969, "$project specification must be an object");
+        }
+        Document projectionSpecification = (Document) projection;
+        if (projectionSpecification.isEmpty()) {
             throw new MongoServerError(40177, "specification must have at least one field");
         }
-        this.projection = projection;
-        this.hasInclusions = hasInclusions(projection);
+        this.projection = projectionSpecification;
+        this.hasInclusions = hasInclusions(projectionSpecification);
         validateProjection();
     }
 
@@ -67,7 +71,7 @@ Document projectDocument(Document document) {
                         Utils.removeSubdocumentValue(result, field);
                     }
                 } else if (projectionValue instanceof List) {
-                    List<Object> resolvedProjectionValues = ((List<Object>) projectionValue)
+                    List<Object> resolvedProjectionValues = ((List<?>) projectionValue)
                         .stream()
                         .map(value -> Expression.evaluateDocument(value, document))
                         .collect(Collectors.toList());

test-common/src/main/java/de/bwaldvogel/mongo/backend/AbstractAggregationTest.java

@@ -630,29 +630,30 @@ void testAggregateWithNestedInclusiveProjection() throws Exception {
             );
     }
 
-    @Test
-    void testAggregateWithIllegalProjection() throws Exception {
-        List<Document> pipeline = jsonList("$project: {'x.b': 1, 'x.c': 1, 'x.d': 0, y: 0}");
+    private static Stream<Arguments> aggregateWithProjectionArguments() {
+        return Stream.of(
+            Arguments.of("$project: {'x.b': 1, 'x.c': 1, 'x.d': 0, y: 0}",
+                "Command failed with error 31254 (Location31254): 'Invalid $project :: caused by :: Cannot do exclusion on field x.d in inclusion projection'"),
 
-        collection.insertOne(json("_id: 1"));
+            Arguments.of("$project: {_id: 0, v: '$x.1.'}",
+                "Command failed with error 40353 (Location40353): 'Invalid $project :: caused by :: FieldPath must not end with a '.'.'"),
 
-        assertThatExceptionOfType(MongoCommandException.class)
-            .isThrownBy(() -> collection.aggregate(pipeline).first())
-            .withMessageContaining("Command failed with error 31254 (Location31254): " +
-                "'Invalid $project :: caused by :: Cannot do exclusion on field x.d in inclusion projection'");
+            Arguments.of("$project: {_id: 0, v: '$x..1'}",
+                "Command failed with error 15998 (Location15998): 'Invalid $project :: caused by :: FieldPath field names may not be empty strings.'"),
+
+            Arguments.of("$project: 'abc'",
+                "Command failed with error 15969 (Location15969): '$project specification must be an object'")
+        );
     }
 
-    @Test
-    void testAggregateWithProjection_IllegalFieldPath() throws Exception {
+    @ParameterizedTest
+    @MethodSource("aggregateWithProjectionArguments")
+    void testAggregateWithProjection_IllegalFieldPath(String project, String expectedMessage) throws Exception {
         collection.insertOne(json("_id: 1, x: 10"));
 
         assertThatExceptionOfType(MongoCommandException.class)
-            .isThrownBy(() -> collection.aggregate(jsonList("$project: {_id: 0, v: '$x.1.'}")).first())
-            .withMessageContaining("Command failed with error 40353 (Location40353): 'Invalid $project :: caused by :: FieldPath must not end with a '.'.'");
-
-        assertThatExceptionOfType(MongoCommandException.class)
-            .isThrownBy(() -> collection.aggregate(jsonList("$project: {_id: 0, v: '$x..1'}")).first())
-            .withMessageContaining("Command failed with error 15998 (Location15998): 'Invalid $project :: caused by :: FieldPath field names may not be empty strings.'");
+            .isThrownBy(() -> collection.aggregate(jsonList(project)).first())
+            .withMessageStartingWith(expectedMessage);
     }
 
     // https://github.com/bwaldvogel/mongo-java-server/pull/189