Favor pipelineStages over toDocument in AggregationOperation.

This commit makes sure to use available pipeline stages to figure out the operation.

Author: christophstrobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationOperation.java

@@ -19,6 +19,7 @@
 import java.util.List;
 
 import org.bson.Document;
+import org.springframework.util.CollectionUtils;
 
 /**
  * Represents one single operation in an aggregation pipeline.
@@ -63,6 +64,6 @@ default List<Document> toPipelineStages(AggregationOperationContext context) {
 	 * @since 3.0.2
 	 */
 	default String getOperator() {
-		return toDocument(Aggregation.DEFAULT_CONTEXT).keySet().iterator().next();
+		return CollectionUtils.lastElement(toPipelineStages(Aggregation.DEFAULT_CONTEXT)).keySet().iterator().next();
 	}
 }

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationOperationUnitTests.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import static org.assertj.core.api.Assertions.*;
+
+import java.util.List;
+
+import org.bson.Document;
+import org.junit.jupiter.api.Test;
+
+/**
+ * @author Christoph Strobl
+ */
+class AggregationOperationUnitTests {
+
+	@Test // GH-4306
+	void getOperatorShouldFavorToPipelineStages() {
+
+		AggregationOperation op = new AggregationOperation() {
+
+			@Override
+			public Document toDocument(AggregationOperationContext context) {
+				return new Document();
+			}
+
+			@Override
+			public List<Document> toPipelineStages(AggregationOperationContext context) {
+				return List.of(new Document("$spring", "data"));
+			}
+		};
+
+		assertThat(op.getOperator()).isEqualTo("$spring");
+	}
+
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationPipelineUnitTests.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.util.List;
+
+import org.bson.Document;
+import org.junit.jupiter.api.Test;
+
+/**
+ * @author Christoph Strobl
+ */
+class AggregationPipelineUnitTests {
+
+	@Test // Gh-4306
+	void verifyMustNotFailIfOnlyPipelineStagesUsed() {
+		AggregationPipeline.of(new OverridesPipelineStagesAndThrowsErrorOnToDocument()).verify();
+	}
+
+	static class OverridesPipelineStagesAndThrowsErrorOnToDocument implements AggregationOperation {
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+			throw new IllegalStateException("oh no");
+		}
+
+		@Override
+		public List<Document> toPipelineStages(AggregationOperationContext context) {
+			return List.of(Aggregation.project("data").toDocument(context));
+		}
+	}
+
+}