DATAMONGO-2363 - Polishing.

Introduce support for SpEL aggregation expressions for AddFields and Set operations.

Rearrange methods. Make fields final where possible.

Original pull request: #801.

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

@@ -151,6 +151,13 @@ public class AddFieldsOperation extends DocumentEnhancingOperation {
 					valueMap.put(field, value instanceof String ? Fields.fields((String) value) : value);
 					return AddFieldsOperationBuilder.this;
 				}
+
+				@Override
+				public AddFieldsOperationBuilder withValueOfExpression(String operation, Object... values) {
+
+					valueMap.put(field, new ExpressionProjection(operation, values));
+					return AddFieldsOperationBuilder.this;
+				}
 			};
 		}
 
@@ -179,7 +186,15 @@ public class AddFieldsOperation extends DocumentEnhancingOperation {
 			 * @return new instance of {@link AddFieldsOperation}.
 			 */
 			AddFieldsOperationBuilder withValueOf(Object value);
-		}
 
+			/**
+			 * Adds a generic projection for the current field.
+			 *
+			 * @param operation the operation key, e.g. {@code $add}.
+			 * @param values the values to be set for the projection operation.
+			 * @return new instance of {@link AddFieldsOperation}.
+			 */
+			AddFieldsOperationBuilder withValueOfExpression(String operation, Object... values);
+		}
 	}
 }

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

@@ -119,7 +119,7 @@ public class Aggregation {
 
 	/**
 	 * Creates a new {@link AggregationUpdate} from the given {@link AggregationOperation}s.
-	 * 
+	 *
 	 * @param operations can be {@literal empty} but must not be {@literal null}.
 	 * @return new instance of {@link AggregationUpdate}.
 	 * @since 3.0
@@ -202,11 +202,16 @@ public class Aggregation {
 		Assert.notNull(aggregationOperations, "AggregationOperations must not be null!");
 		Assert.notNull(options, "AggregationOptions must not be null!");
 
-		// check $out is the last operation if it exists
+		// check $out/$merge is the last operation if it exists
 		for (AggregationOperation aggregationOperation : aggregationOperations) {
+
 			if (aggregationOperation instanceof OutOperation && !isLast(aggregationOperation, aggregationOperations)) {
 				throw new IllegalArgumentException("The $out operator must be the last stage in the pipeline.");
 			}
+
+			if (aggregationOperation instanceof MergeOperation && !isLast(aggregationOperation, aggregationOperations)) {
+				throw new IllegalArgumentException("The $merge operator must be the last stage in the pipeline.");
+			}
 		}
 
 		this.operations = aggregationOperations;
@@ -236,6 +241,20 @@ public class Aggregation {
 		return "_id";
 	}
 
+	/**
+	 * Obtain an {@link AddFieldsOperationBuilder builder} instance to create a new {@link AddFieldsOperation}.
+	 * <p/>
+	 * Starting in version 4.2, MongoDB adds a new aggregation pipeline stage {@link AggregationUpdate#set $set} that is
+	 * an alias for {@code $addFields}.
+	 *
+	 * @return new instance of {@link AddFieldsOperationBuilder}.
+	 * @see AddFieldsOperation
+	 * @since 3.0
+	 */
+	public static AddFieldsOperationBuilder addFields() {
+		return AddFieldsOperation.builder();
+	}
+
 	/**
 	 * Creates a new {@link ProjectionOperation} including the given fields.
 	 *
@@ -495,6 +514,30 @@ public class Aggregation {
 		return new MatchOperation(criteria);
 	}
 
+	/**
+	 * Creates a new {@link GeoNearOperation} instance from the given {@link NearQuery} and the {@code distanceField}. The
+	 * {@code distanceField} defines output field that contains the calculated distance.
+	 *
+	 * @param query must not be {@literal null}.
+	 * @param distanceField must not be {@literal null} or empty.
+	 * @return
+	 * @since 1.7
+	 */
+	public static GeoNearOperation geoNear(NearQuery query, String distanceField) {
+		return new GeoNearOperation(query, distanceField);
+	}
+
+	/**
+	 * Obtain a {@link MergeOperationBuilder builder} instance to create a new {@link MergeOperation}.
+	 *
+	 * @return new instance of {@link MergeOperationBuilder}.
+	 * @see MergeOperation
+	 * @since 3.0
+	 */
+	public static MergeOperationBuilder merge() {
+		return MergeOperation.builder();
+	}
+
 	/**
 	 * Creates a new {@link OutOperation} using the given collection name. This operation must be the last operation in
 	 * the pipeline.
@@ -636,41 +679,6 @@ public class Aggregation {
 		return Fields.from(field(name, target));
 	}
 
-	/**
-	 * Creates a new {@link GeoNearOperation} instance from the given {@link NearQuery} and the {@code distanceField}. The
-	 * {@code distanceField} defines output field that contains the calculated distance.
-	 *
-	 * @param query must not be {@literal null}.
-	 * @param distanceField must not be {@literal null} or empty.
-	 * @return
-	 * @since 1.7
-	 */
-	public static GeoNearOperation geoNear(NearQuery query, String distanceField) {
-		return new GeoNearOperation(query, distanceField);
-	}
-
-	/**
-	 * Obtain a {@link MergeOperationBuilder builder} instance to create a new {@link MergeOperation}.
-	 *
-	 * @return new instance of {@link MergeOperationBuilder}.
-	 * @see MergeOperation
-	 * @since 3.0
-	 */
-	public static MergeOperationBuilder merge() {
-		return MergeOperation.builder();
-	}
-
-	/**
-	 * Obtain an {@link AddFieldsOperationBuilder builder} instance to create a new {@link AddFieldsOperation}.
-	 * 
-	 * @return new instance of {@link AddFieldsOperationBuilder}.
-	 * @see AddFieldsOperation
-	 * @since 3.0
-	 */
-	public static AddFieldsOperationBuilder addFields() {
-		return AddFieldsOperation.builder();
-	}
-
 	/**
 	 * Returns a new {@link AggregationOptions.Builder}.
 	 *

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

@@ -79,7 +79,7 @@ import org.springframework.util.Assert;
 public class AggregationUpdate extends Aggregation implements UpdateDefinition {
 
 	private boolean isolated = false;
-	private Set<String> keysTouched = new HashSet<>();
+	private final Set<String> keysTouched = new HashSet<>();
 
 	/**
 	 * Create new {@link AggregationUpdate}.

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

@@ -24,16 +24,18 @@ import java.util.stream.Collectors;
 import org.bson.Document;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
 import org.springframework.data.mongodb.core.aggregation.FieldsExposingAggregationOperation.InheritsFieldsAggregationOperation;
+import org.springframework.util.Assert;
 
 /**
- * Base class for common taks required by {@link SetOperation} and {@link AddFieldsOperation}.
+ * Base class for common tasks required by {@link SetOperation} and {@link AddFieldsOperation}.
  *
  * @author Christoph Strobl
  * @since 3.0
  */
 abstract class DocumentEnhancingOperation implements InheritsFieldsAggregationOperation {
 
-	private Map<Object, Object> valueMap;
+	private final Map<Object, Object> valueMap;
+
 	private ExposedFields exposedFields = ExposedFields.empty();
 
 	protected DocumentEnhancingOperation(Map<Object, Object> source) {
@@ -112,14 +114,53 @@ abstract class DocumentEnhancingOperation implements InheritsFieldsAggregationOp
 		if (value instanceof Field) {
 			return context.getReference((Field) value).toString();
 		}
+
+		if (value instanceof ExpressionProjection) {
+			return ((ExpressionProjection) value).toExpression(context);
+		}
+
 		if (value instanceof AggregationExpression) {
 			return ((AggregationExpression) value).toDocument(context);
 		}
+
 		if (value instanceof Collection) {
-			return ((Collection) value).stream().map(it -> computeValue(it, context)).collect(Collectors.toList());
+			return ((Collection<?>) value).stream().map(it -> computeValue(it, context)).collect(Collectors.toList());
 		}
 
 		return value;
 	}
 
+	/**
+	 * A {@link AggregationExpression} based on a SpEL expression.
+	 *
+	 * @author Mark Paluch
+	 */
+	static class ExpressionProjection {
+
+		private static final SpelExpressionTransformer TRANSFORMER = new SpelExpressionTransformer();
+
+		private final String expression;
+		private final Object[] params;
+
+		/**
+		 * Creates a new {@link ProjectionOperation.ExpressionProjectionOperationBuilder.ExpressionProjection} for the given
+		 * field, SpEL expression and parameters.
+		 *
+		 * @param expression must not be {@literal null} or empty.
+		 * @param parameters must not be {@literal null}.
+		 */
+		ExpressionProjection(String expression, Object[] parameters) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			Assert.notNull(parameters, "Parameters must not be null!");
+
+			this.expression = expression;
+			this.params = parameters.clone();
+		}
+
+		Object toExpression(AggregationOperationContext context) {
+			return TRANSFORMER.transform(expression, context, params);
+		}
+	}
+
 }

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

@@ -33,8 +33,8 @@ import org.springframework.util.StringUtils;
 /**
  * Encapsulates the {@code $merge}-operation.
  * <p>
- * We recommend to use the {@link MergeOperationBuilder builder} via {@link MergeOperation#builder()} instead of creating
- * instances of this class directly.
+ * We recommend to use the {@link MergeOperationBuilder builder} via {@link MergeOperation#builder()} instead of
+ * creating instances of this class directly.
  *
  * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/merge/">MongoDB Documentation</a>
  * @author Christoph Strobl
@@ -42,15 +42,15 @@ import org.springframework.util.StringUtils;
  */
 public class MergeOperation implements FieldsExposingAggregationOperation, InheritsFieldsAggregationOperation {
 
-	private MergeOperationTarget into;
-	private UniqueMergeId on;
-	private @Nullable Let let;
-	private @Nullable WhenDocumentsMatch whenMatched;
-	private @Nullable WhenDocumentsDontMatch whenNotMatched;
+	private final MergeOperationTarget into;
+	private final UniqueMergeId on;
+	private final @Nullable Let let;
+	private final @Nullable WhenDocumentsMatch whenMatched;
+	private final @Nullable WhenDocumentsDontMatch whenNotMatched;
 
 	/**
 	 * Create new instance of {@link MergeOperation}.
-	 * 
+	 *
 	 * @param into the target (collection and database)
 	 * @param on the unique identifier. Can be {@literal null}.
 	 * @param let exposed variables for {@link WhenDocumentsMatch#updateWith(Aggregation)}. Can be {@literal null}.
@@ -63,7 +63,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 			@Nullable WhenDocumentsMatch whenMatched, @Nullable WhenDocumentsDontMatch whenNotMatched) {
 
 		Assert.notNull(into, "Into must not be null! Please provide a target collection.");
-		Assert.notNull(on, "On must not be null! Use Collections.emptySet() instead.");
+		Assert.notNull(on, "On must not be null! Use UniqueMergeId.id() instead.");
 
 		this.into = into;
 		this.on = on;
@@ -93,7 +93,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 	}
 
 	/*
-	 * (non-Javadoc) 
+	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.aggregation.Aggregation#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
 	 */
 	@Override
@@ -109,15 +109,17 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 		if (!on.isJustIdField()) {
 			$merge.putAll(on.toDocument(context));
 		}
+
 		if (let != null) {
 			$merge.append("let", let.toDocument(context).get("$let", Document.class).get("vars"));
 		}
+
 		if (whenMatched != null) {
 			$merge.putAll(whenMatched.toDocument(context));
 		}
+
 		if (whenNotMatched != null) {
 			$merge.putAll(whenNotMatched.toDocument(context));
-
 		}
 
 		return new Document("$merge", $merge);
@@ -159,13 +161,12 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 	 * collection.
 	 *
 	 * @author Christoph Strobl
-	 * @since 2.3
 	 */
 	public static class UniqueMergeId {
 
 		private static final UniqueMergeId ID = new UniqueMergeId(Collections.emptyList());
 
-		private Collection<String> uniqueIdentifier;
+		private final Collection<String> uniqueIdentifier;
 
 		private UniqueMergeId(Collection<String> uniqueIdentifier) {
 			this.uniqueIdentifier = uniqueIdentifier;
@@ -173,6 +174,8 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 
 		public static UniqueMergeId ofIdFields(String... fields) {
 
+			Assert.noNullElements(fields, "Fields must not contain null values!");
+
 			if (ObjectUtils.isEmpty(fields)) {
 				return id();
 			}
@@ -182,7 +185,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 
 		/**
 		 * Merge Documents by using the MongoDB {@literal _id} field.
-		 * 
+		 *
 		 * @return
 		 */
 		public static UniqueMergeId id() {
@@ -206,19 +209,19 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 	 * If not stated explicitly via {@link MergeOperationTarget#inDatabase(String)} the {@literal collection} is created
 	 * in the very same {@literal database}. In this case {@code into} is just a single String holding the collection
 	 * name. <br />
-	 * 
+	 *
 	 * <pre class="code">
 	 *     into: "target-collection-name"
 	 * </pre>
-	 * 
+	 *
 	 * If the collection needs to be in a different database {@code into} will be a {@link Document} like the following
-	 * 
+	 *
 	 * <pre class="code">
 	 * {
 	 * 	into: {}
 	 * }
 	 * </pre>
-	 * 
+	 *
 	 * @author Christoph Strobl
 	 * @since 2.3
 	 */
@@ -267,7 +270,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 	/**
 	 * Value Object specifying how to deal with a result document that matches an existing document in the collection
 	 * based on the fields of the {@code on} property describing the unique identifier.
-	 * 
+	 *
 	 * @author Christoph Strobl
 	 * @since 2.3
 	 */
@@ -354,7 +357,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 	/**
 	 * Value Object specifying how to deal with a result document that do not match an existing document in the collection
 	 * based on the fields of the {@code on} property describing the unique identifier.
-	 * 
+	 *
 	 * @author Christoph Strobl
 	 * @since 2.3
 	 */
@@ -363,9 +366,18 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 		private final String value;
 
 		private WhenDocumentsDontMatch(String value) {
+
+			Assert.notNull(value, "Value must not be null!");
+
 			this.value = value;
 		}
 
+		/**
+		 * Factory method creating {@link WhenDocumentsDontMatch} from a {@code value} literal.
+		 *
+		 * @param value
+		 * @return new instance of {@link WhenDocumentsDontMatch}.
+		 */
 		public static WhenDocumentsDontMatch whenNotMatchedOf(String value) {
 			return new WhenDocumentsDontMatch(value);
 		}
@@ -412,7 +424,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 
 		private String collection;
 		private @Nullable String database;
-		private @Nullable UniqueMergeId id;
+		private UniqueMergeId id = UniqueMergeId.id();
 		private @Nullable Let let;
 		private @Nullable WhenDocumentsMatch whenMatched;
 		private @Nullable WhenDocumentsDontMatch whenNotMatched;
@@ -447,7 +459,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 
 		/**
 		 * Define the target to store results in.
-		 * 
+		 *
 		 * @param into must not be {@literal null}.
 		 * @return this.
 		 */
@@ -484,7 +496,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 		/**
 		 * Set the identifier that determines if a results document matches an already existing document in the output
 		 * collection.
-		 * 
+		 *
 		 * @param id must not be {@literal null}.
 		 * @return this.
 		 */
@@ -497,7 +509,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 		/**
 		 * Expose the variables defined by {@link Let} to the {@link WhenDocumentsMatch#updateWith(Aggregation) update
 		 * aggregation}.
-		 * 
+		 *
 		 * @param let the variable expressions
 		 * @return this.
 		 */
@@ -520,7 +532,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 
 		/**
 		 * The action to take place when documents already exist in the target collection.
-		 * 
+		 *
 		 * @param whenMatched must not be {@literal null}.
 		 * @return this.
 		 */
@@ -576,8 +588,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 		 * @return new instance of {@link MergeOperation}.
 		 */
 		public MergeOperation build() {
-			return new MergeOperation(new MergeOperationTarget(database, collection), id != null ? id : UniqueMergeId.id(),
-					let, whenMatched, whenNotMatched);
+			return new MergeOperation(new MergeOperationTarget(database, collection), id, let, whenMatched, whenNotMatched);
 		}
 	}
 }

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

@@ -25,7 +25,7 @@ import org.springframework.lang.Nullable;
 /**
  * Adds new fields to documents. {@code $set} outputs documents that contain all existing fields from the input
  * documents and newly added fields.
- * 
+ *
  * <pre class="code">
  * SetOperation.set("totalHomework").toValue("A+").and().set("totalQuiz").toValue("B-")
  * </pre>
@@ -143,6 +143,13 @@ public class SetOperation extends DocumentEnhancingOperation {
 					valueMap.put(field, value instanceof String ? Fields.fields((String) value) : value);
 					return FieldAppender.this.build();
 				}
+
+				@Override
+				public SetOperation withValueOfExpression(String operation, Object... values) {
+
+					valueMap.put(field, new ExpressionProjection(operation, values));
+					return FieldAppender.this.build();
+				}
 			};
 		}
 
@@ -152,6 +159,7 @@ public class SetOperation extends DocumentEnhancingOperation {
 
 		/**
 		 * @author Christoph Strobl
+		 * @author Mark Paluch
 		 * @since 3.0
 		 */
 		public interface ValueAppender {
@@ -171,6 +179,15 @@ public class SetOperation extends DocumentEnhancingOperation {
 			 * @return new instance of {@link SetOperation}.
 			 */
 			SetOperation toValueOf(Object value);
+
+			/**
+			 * Adds a generic projection for the current field.
+			 *
+			 * @param operation the operation key, e.g. {@code $add}.
+			 * @param values the values to be set for the projection operation.
+			 * @return new instance of {@link SetOperation}.
+			 */
+			SetOperation withValueOfExpression(String operation, Object... values);
 		}
 	}
 }

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

@@ -29,7 +29,10 @@ import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
 import org.springframework.lang.Nullable;
 
 /**
+ * Unit tests for {@link AddFieldsOperation}.
+ *
  * @author Christoph Strobl
+ * @author Mark Paluch
  */
 class AddFieldsOperationUnitTests {
 
@@ -92,6 +95,17 @@ class AddFieldsOperationUnitTests {
 								"{\"$addFields\" : {\"scoresWithMappedField.student_name\":\"$scoresWithMappedField.home_work\"}}"));
 	}
 
+	@Test // DATAMONGO-2363
+	void appliesSpelExpressionCorrectly() {
+
+		AddFieldsOperation operation = AddFieldsOperation.builder().addField("totalHomework")
+				.withValueOfExpression("sum(homework) * [0]", 2) //
+				.build();
+
+		assertThat(operation.toPipelineStages(contextFor(ScoresWrapper.class))).contains(
+				Document.parse("{\"$addFields\" : {\"totalHomework\": { $multiply : [{ \"$sum\" : [\"$homework\"] }, 2] }}}"));
+	}
+
 	@Test // DATAMONGO-2363
 	void rendersTargetValueExpressionCorrectly() {
 
@@ -105,10 +119,11 @@ class AddFieldsOperationUnitTests {
 
 		ExposedFields fields = AddFieldsOperation.builder().addField("totalHomework").withValue("A+") //
 				.addField("totalQuiz").withValue("B-") //
-				.build().getFields();
+				.addField("computed").withValueOfExpression("totalHomework").build().getFields();
 
 		assertThat(fields.getField("totalHomework")).isNotNull();
 		assertThat(fields.getField("totalQuiz")).isNotNull();
+		assertThat(fields.getField("computed")).isNotNull();
 		assertThat(fields.getField("does-not-exist")).isNull();
 	}
 

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

@@ -33,6 +33,8 @@ import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
 import org.springframework.lang.Nullable;
 
 /**
+ * Unit tests for {@link MergeOperation}.
+ *
  * @author Christoph Strobl
  */
 class MergeOperationUnitTests {
@@ -98,7 +100,7 @@ class MergeOperationUnitTests {
 	}
 
 	@Test // DATAMONGO-2363
-	public void mapsFieldNames() {
+	void mapsFieldNames() {
 
 		assertThat(merge().intoCollection("newrestaurants").on("date", "postCode").build()
 				.toDocument(contextFor(Restaurant.class))).isEqualTo(

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

@@ -32,30 +32,31 @@ import org.springframework.lang.Nullable;
  * Unit tests for {@link SetOperation}.
  *
  * @author Christoph Strobl
+ * @author Mark Paluch
  */
-public class SetOperationUnitTests {
+class SetOperationUnitTests {
 
 	@Test // DATAMONGO-2331
-	public void raisesErrorOnNullField() {
+	void raisesErrorOnNullField() {
 		assertThatExceptionOfType(IllegalArgumentException.class).isThrownBy(() -> new SetOperation(null, "value"));
 	}
 
 	@Test // DATAMONGO-2331
-	public void rendersFieldReferenceCorrectly() {
+	void rendersFieldReferenceCorrectly() {
 
 		assertThat(new SetOperation("name", "value").toPipelineStages(contextFor(Scores.class)))
 				.containsExactly(Document.parse("{\"$set\" : {\"name\":\"value\"}}"));
 	}
 
 	@Test // DATAMONGO-2331
-	public void rendersMappedFieldReferenceCorrectly() {
+	void rendersMappedFieldReferenceCorrectly() {
 
 		assertThat(new SetOperation("student", "value").toPipelineStages(contextFor(ScoresWithMappedField.class)))
 				.containsExactly(Document.parse("{\"$set\" : {\"student_name\":\"value\"}}"));
 	}
 
 	@Test // DATAMONGO-2331
-	public void rendersNestedMappedFieldReferenceCorrectly() {
+	void rendersNestedMappedFieldReferenceCorrectly() {
 
 		assertThat(
 				new SetOperation("scoresWithMappedField.student", "value").toPipelineStages(contextFor(ScoresWrapper.class)))
@@ -63,14 +64,14 @@ public class SetOperationUnitTests {
 	}
 
 	@Test // DATAMONGO-2331
-	public void rendersTargetValueFieldReferenceCorrectly() {
+	void rendersTargetValueFieldReferenceCorrectly() {
 
 		assertThat(new SetOperation("name", Fields.field("value")).toPipelineStages(contextFor(Scores.class)))
 				.containsExactly(Document.parse("{\"$set\" : {\"name\":\"$value\"}}"));
 	}
 
 	@Test // DATAMONGO-2331
-	public void rendersMappedTargetValueFieldReferenceCorrectly() {
+	void rendersMappedTargetValueFieldReferenceCorrectly() {
 
 		assertThat(
 				new SetOperation("student", Fields.field("homework")).toPipelineStages(contextFor(ScoresWithMappedField.class)))
@@ -78,7 +79,7 @@ public class SetOperationUnitTests {
 	}
 
 	@Test // DATAMONGO-2331
-	public void rendersNestedMappedTargetValueFieldReferenceCorrectly() {
+	void rendersNestedMappedTargetValueFieldReferenceCorrectly() {
 
 		assertThat(new SetOperation("scoresWithMappedField.student", Fields.field("scoresWithMappedField.homework"))
 				.toPipelineStages(contextFor(ScoresWrapper.class)))
@@ -86,8 +87,18 @@ public class SetOperationUnitTests {
 								.parse("{\"$set\" : {\"scoresWithMappedField.student_name\":\"$scoresWithMappedField.home_work\"}}"));
 	}
 
+	@Test // DATAMONGO-2363
+	void appliesSpelExpressionCorrectly() {
+
+		SetOperation operation = SetOperation.builder().set("totalHomework").withValueOfExpression("sum(homework) * [0]",
+				2);
+
+		assertThat(operation.toPipelineStages(contextFor(AddFieldsOperationUnitTests.ScoresWrapper.class))).contains(
+				Document.parse("{\"$set\" : {\"totalHomework\": { $multiply : [{ \"$sum\" : [\"$homework\"] }, 2] }}}"));
+	}
+
 	@Test // DATAMONGO-2331
-	public void rendersTargetValueExpressionCorrectly() {
+	void rendersTargetValueExpressionCorrectly() {
 
 		assertThat(SetOperation.builder().set("totalHomework").toValueOf(ArithmeticOperators.valueOf("homework").sum())
 				.toPipelineStages(contextFor(Scores.class)))
@@ -95,7 +106,7 @@ public class SetOperationUnitTests {
 	}
 
 	@Test // DATAMONGO-2331
-	public void exposesFieldsCorrectly() {
+	void exposesFieldsCorrectly() {
 
 		ExposedFields fields = SetOperation.builder().set("totalHomework").toValue("A+") //
 				.and() //
@@ -138,5 +149,4 @@ public class SetOperationUnitTests {
 		Scores scores;
 		ScoresWithMappedField scoresWithMappedField;
 	}
-
 }