Include & Exclude paths in project aggregation stage.

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

@@ -167,6 +167,46 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		return new ProjectionOperation(this.projections, projections);
 	}
 
+	/**
+	 * @param path
+	 * @return
+	 * @since 4.2
+	 */
+	public ProjectionOperation andIncludePath(String path) {
+		return andIncludePaths(new String[] { path });
+	}
+
+	/**
+	 * @param paths
+	 * @return
+	 * @since 4.2
+	 */
+	public ProjectionOperation andIncludePaths(String... paths) {
+
+		List<FieldProjection> projections = FieldProjection.from(Fields.fields(paths), 1);
+		return new ProjectionOperation(this.projections, projections);
+	}
+
+	/**
+	 * @param path
+	 * @return
+	 * @since 4.2
+	 */
+	public ProjectionOperation andExcludePath(String path) {
+		return andExcludePaths(new String[] { path });
+	}
+
+	/**
+	 * @param paths
+	 * @return
+	 * @since 4.2
+	 */
+	public ProjectionOperation andExcludePaths(String... paths) {
+
+		List<FieldProjection> projections = FieldProjection.from(Fields.fields(paths), 0);
+		return new ProjectionOperation(this.projections, projections);
+	}
+
 	/**
 	 * Includes the given fields into the projection.
 	 *
@@ -495,8 +535,8 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 			if (value instanceof AggregationExpression) {
 				return this.operation.and(new ExpressionProjection(Fields.field(alias, alias), (AggregationExpression) value));
 			}
-
-			return this.operation.and(new FieldProjection(Fields.field(alias, getRequiredName()), null));
+			Field field = Fields.field(alias, getRequiredName());
+			return this.operation.and(new FieldProjection(field, null));
 		}
 
 		@Override
@@ -1329,6 +1369,11 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 
 			private final Field field;
 			private final @Nullable Object value;
+			private final ProjectOn projectOn;
+
+			enum ProjectOn {
+				PATH, NAME
+			}
 
 			/**
 			 * Creates a new {@link FieldProjection} for the field of the given name, assigning the given value.
@@ -1341,11 +1386,16 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 			}
 
 			private FieldProjection(Field field, @Nullable Object value) {
+				this(field, value, value instanceof Integer ? ProjectOn.PATH : ProjectOn.NAME);
+			}
+
+			private FieldProjection(Field field, @Nullable Object value, ProjectOn project) {
 
 				super(new ExposedField(field.getName(), true));
 
 				this.field = field;
 				this.value = value;
+				this.projectOn = project;
 			}
 
 			/**
@@ -1372,7 +1422,8 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 				List<FieldProjection> projections = new ArrayList<FieldProjection>();
 
 				for (Field field : fields) {
-					projections.add(new FieldProjection(field, value));
+					projections
+							.add(new FieldProjection(field, value, value instanceof Integer ? ProjectOn.PATH : ProjectOn.NAME));
 				}
 
 				return projections;
@@ -1382,12 +1433,13 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 			 * @return {@literal true} if this field is excluded.
 			 */
 			public boolean isExcluded() {
-				return Boolean.FALSE.equals(value);
+				return Boolean.FALSE.equals(value) || value instanceof Number number && number.intValue() == 0;
 			}
 
 			@Override
 			public Document toDocument(AggregationOperationContext context) {
-				return new Document(field.getName(), renderFieldValue(context));
+				return new Document(ProjectOn.NAME.equals(projectOn) ? field.getName() : context.getReference(field.getTarget()).getRaw(),
+						renderFieldValue(context));
 			}
 
 			private Object renderFieldValue(AggregationOperationContext context) {

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

@@ -36,6 +36,7 @@ import org.springframework.data.mongodb.core.aggregation.ProjectionOperationUnit
 import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
 import org.springframework.data.mongodb.core.convert.NoOpDbRefResolver;
 import org.springframework.data.mongodb.core.convert.QueryMapper;
+import org.springframework.data.mongodb.core.mapping.Field;
 import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
 import org.springframework.data.mongodb.core.query.Criteria;
 
@@ -653,6 +654,58 @@ public class AggregationUnitTests {
 		assertThat(documents.get(2)).isEqualTo("{ $sort : { 'serial_number' : -1, 'label_name' : -1 } }");
 	}
 
+	@Test // GH-4428
+	void projectIncludePath() {
+
+		MongoMappingContext mappingContext = new MongoMappingContext();
+		RelaxedTypeBasedAggregationOperationContext context = new RelaxedTypeBasedAggregationOperationContext(
+				Root.class, mappingContext,
+				new QueryMapper(new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext)));
+
+		assertThat(project("flat").andIncludePath("list.element").toDocument(context)).isEqualTo(
+				Document.parse("""
+						{
+						  "$project": {
+						    "flat": 1,
+						    "list.elE_m_enT": 1
+						  }
+						}""")
+		);
+	}
+
+	@Test // GH-4428
+	void projectExcludePath() {
+
+		MongoMappingContext mappingContext = new MongoMappingContext();
+		RelaxedTypeBasedAggregationOperationContext context = new RelaxedTypeBasedAggregationOperationContext(
+				Root.class, mappingContext,
+				new QueryMapper(new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext)));
+
+		assertThat(project("flat").andExcludePath("list.element").toDocument(context)).isEqualTo(
+				Document.parse("""
+						{
+						  "$project": {
+						    "flat": 1,
+						    "list.elE_m_enT": 0
+						  }
+						}""")
+		);
+	}
+
+
+	static class Root {
+		String flat;
+		List<Nested> list;
+	}
+
+	static class Nested {
+
+		@Field("elE_m_enT")
+		int element;
+		String description;
+
+	}
+
 	private Document extractPipelineElement(Document agg, int index, String operation) {
 
 		List<Document> pipeline = (List<Document>) agg.get("pipeline");