Introduce dedicated type representing variables within an aggregation.

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-mongodb-parent</artifactId>
-	<version>4.1.0-SNAPSHOT</version>
+	<version>4.1.x-GH-4070-SNAPSHOT</version>
 	<packaging>pom</packaging>
 
 	<name>Spring Data MongoDB</name>

spring-data-mongodb-benchmarks/pom.xml

@@ -7,7 +7,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>4.1.0-SNAPSHOT</version>
+		<version>4.1.x-GH-4070-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb-distribution/pom.xml

@@ -15,7 +15,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>4.1.0-SNAPSHOT</version>
+		<version>4.1.x-GH-4070-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/pom.xml

@@ -13,7 +13,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>4.1.0-SNAPSHOT</version>
+		<version>4.1.x-GH-4070-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

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

@@ -0,0 +1,130 @@
+/*
+ * Copyright 2022 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 org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
+
+/**
+ * A special field that points to a variable {@code $$} expression.
+ *
+ * @author Christoph Strobl
+ * @since 4.1
+ */
+public interface AggregationVariable extends Field {
+
+	String PREFIX = "$$";
+
+	/**
+	 * @return {@literal true} if the fields {@link #getName() name} does not match the defined {@link #getTarget()
+	 *         target}.
+	 */
+	default boolean isAliased() {
+		return !ObjectUtils.nullSafeEquals(getName(), getTarget());
+	}
+
+	@Override
+	default String getName() {
+		return getTarget();
+	}
+
+	default boolean isInternal() {
+		return false;
+	}
+
+	/**
+	 * Create a new {@link AggregationVariable} for the given name.
+	 * <p>
+	 * Variables start with {@code $$}. If not, the given value gets prefixed with {@code $$}.
+	 * 
+	 * @param value must not be {@literal null}.
+	 * @return new instance of {@link AggregationVariable}.
+	 * @throws IllegalArgumentException if given value is {@literal null}.
+	 */
+	static AggregationVariable variable(String value) {
+
+		Assert.notNull(value, "Value must not be null");
+		return new AggregationVariable() {
+
+			private final String val = AggregationVariable.prefixVariable(value);
+
+			@Override
+			public String getTarget() {
+				return val;
+			}
+		};
+	}
+
+	/**
+	 * Create a new {@link #isInternal() local} {@link AggregationVariable} for the given name.
+	 * <p>
+	 * Variables start with {@code $$}. If not, the given value gets prefixed with {@code $$}.
+	 *
+	 * @param value must not be {@literal null}.
+	 * @return new instance of {@link AggregationVariable}.
+	 * @throws IllegalArgumentException if given value is {@literal null}.
+	 */
+	static AggregationVariable localVariable(String value) {
+
+		Assert.notNull(value, "Value must not be null");
+		return new AggregationVariable() {
+
+			private final String val = AggregationVariable.prefixVariable(value);
+
+			@Override
+			public String getTarget() {
+				return val;
+			}
+
+			@Override
+			public boolean isInternal() {
+				return true;
+			}
+		};
+	}
+
+	/**
+	 * Check if the given field name reference may be variable.
+	 *
+	 * @param fieldRef can be {@literal null}.
+	 * @return true if given value matches the variable identification pattern.
+	 */
+	static boolean isVariable(@Nullable String fieldRef) {
+		return fieldRef != null && fieldRef.stripLeading().matches("^\\$\\$\\w.*");
+	}
+
+	/**
+	 * Check if the given field may be variable.
+	 *
+	 * @param field can be {@literal null}.
+	 * @return true if given {@link Field field} is an {@link AggregationVariable} or if its value is a
+	 *         {@link #isVariable(String) variable}.
+	 */
+	static boolean isVariable(Field field) {
+
+		if (field instanceof AggregationVariable) {
+			return true;
+		}
+		return isVariable(field.getTarget());
+	}
+
+	private static String prefixVariable(String variable) {
+
+		var trimmed = variable.stripLeading();
+		return trimmed.startsWith(PREFIX) ? trimmed : (PREFIX + trimmed);
+	}
+}

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

@@ -1471,52 +1471,39 @@ public interface AsBuilder {
 			}
 		}
 
-		public enum Variable implements Field {
+		public enum Variable implements AggregationVariable {
 
 			THIS {
-				@Override
-				public String getName() {
-					return "$$this";
-				}
 
 				@Override
 				public String getTarget() {
 					return "$$this";
 				}
 
-				@Override
-				public boolean isAliased() {
-					return false;
-				}
-
 				@Override
 				public String toString() {
 					return getName();
 				}
 			},
 
 			VALUE {
-				@Override
-				public String getName() {
-					return "$$value";
-				}
 
 				@Override
 				public String getTarget() {
 					return "$$value";
 				}
 
-				@Override
-				public boolean isAliased() {
-					return false;
-				}
-
 				@Override
 				public String toString() {
 					return getName();
 				}
 			};
 
+			@Override
+			public boolean isInternal() {
+				return true;
+			}
+
 			/**
 			 * Create a {@link Field} reference to a given {@literal property} prefixed with the {@link Variable} identifier.
 			 * eg. {@code $$value.product}
@@ -1548,6 +1535,16 @@ public String toString() {
 					}
 				};
 			}
+
+			public static boolean isVariable(Field field) {
+
+				for (Variable var : values()) {
+					if (field.getTarget().startsWith(var.getTarget())) {
+						return true;
+					}
+				}
+				return false;
+			}
 		}
 	}
 

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

@@ -245,7 +245,7 @@ public AggregationField(String name, @Nullable String target) {
 
 		private static String cleanUp(String source) {
 
-			if (SystemVariable.isReferingToSystemVariable(source)) {
+			if (AggregationVariable.isVariable(source)) {
 				return source;
 			}
 

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

@@ -24,7 +24,7 @@
  * @author Christoph Strobl
  * @see <a href="https://docs.mongodb.com/manual/reference/aggregation-variables">Aggregation Variables</a>.
  */
-public enum SystemVariable {
+public enum SystemVariable implements AggregationVariable {
 
 	/**
 	 * Variable for the current datetime.
@@ -82,8 +82,6 @@ public enum SystemVariable {
 	 */
 	SEARCH_META;
 
-	private static final String PREFIX = "$$";
-
 	/**
 	 * Return {@literal true} if the given {@code fieldRef} denotes a well-known system variable, {@literal false}
 	 * otherwise.
@@ -93,13 +91,12 @@ public enum SystemVariable {
 	 */
 	public static boolean isReferingToSystemVariable(@Nullable String fieldRef) {
 
-		if (fieldRef == null || !fieldRef.startsWith(PREFIX) || fieldRef.length() <= 2) {
+		String candidate = variableNameFrom(fieldRef);
+		if (candidate == null) {
 			return false;
 		}
 
-		int indexOfFirstDot = fieldRef.indexOf('.');
-		String candidate = fieldRef.substring(2, indexOfFirstDot == -1 ? fieldRef.length() : indexOfFirstDot);
-
+		candidate = candidate.startsWith(PREFIX) ? candidate.substring(2) : candidate;
 		for (SystemVariable value : values()) {
 			if (value.name().equals(candidate)) {
 				return true;
@@ -113,4 +110,20 @@ public static boolean isReferingToSystemVariable(@Nullable String fieldRef) {
 	public String toString() {
 		return PREFIX.concat(name());
 	}
+
+	@Override
+	public String getTarget() {
+		return toString();
+	}
+
+	@Nullable
+	static String variableNameFrom(@Nullable String fieldRef) {
+
+		if (fieldRef == null || !fieldRef.startsWith(PREFIX) || fieldRef.length() <= 2) {
+			return null;
+		}
+
+		int indexOfFirstDot = fieldRef.indexOf('.');
+		return indexOfFirstDot == -1 ? fieldRef : fieldRef.substring(2, indexOfFirstDot);
+	}
 }

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

@@ -133,7 +133,7 @@ public AggregationOperationContext continueOnMissingFieldReference(Class<?> type
 
 	protected FieldReference getReferenceFor(Field field) {
 
-		if(entity.getNullable() == null) {
+		if(entity.getNullable() == null || AggregationVariable.isVariable(field)) {
 			return new DirectFieldReference(new ExposedField(field, true));
 		}