Support expressions in query field projections.

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-mongodb-parent</artifactId>
-	<version>3.2.0-SNAPSHOT</version>
+	<version>3.2.0-GH-3583-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>3.2.0-SNAPSHOT</version>
+		<version>3.2.0-GH-3583-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb-distribution/pom.xml

@@ -14,7 +14,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.2.0-SNAPSHOT</version>
+		<version>3.2.0-GH-3583-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/pom.xml

@@ -11,7 +11,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.2.0-SNAPSHOT</version>
+		<version>3.2.0-GH-3583-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/BindableMongoExpression.java

@@ -0,0 +1,151 @@
+/*
+ * Copyright 2021 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;
+
+import org.bson.Document;
+import org.bson.codecs.DocumentCodec;
+import org.bson.codecs.configuration.CodecRegistry;
+import org.springframework.data.mongodb.util.json.ParameterBindingDocumentCodec;
+import org.springframework.data.util.Lazy;
+import org.springframework.lang.Nullable;
+import org.springframework.util.ObjectUtils;
+import org.springframework.util.StringUtils;
+
+/**
+ * A {@link MongoExpression} using the {@link ParameterBindingDocumentCodec} for parsing a raw ({@literal json})
+ * expression. The expression will be wrapped within <code>{ ... }</code> if necessary. The actual parsing and parameter
+ * binding of placeholders like {@code ?0} is delayed upon first call on the the target {@link Document} via
+ * {@link #toDocument()}.
+ * <p />
+ * 
+ * <pre class="code">
+ * $toUpper : $name                -> { '$toUpper' : '$name' }
+ * 
+ * { '$toUpper' : '$name' }        -> { '$toUpper' : '$name' }
+ * 
+ * { '$toUpper' : '?0' }, "$name"  -> { '$toUpper' : '$name' }
+ * </pre>
+ * 
+ * Some types might require a special {@link org.bson.codecs.Codec}. If so, make sure to provide a {@link CodecRegistry}
+ * containing the required {@link org.bson.codecs.Codec codec} via {@link #withCodecRegistry(CodecRegistry)}.
+ *
+ * @author Christoph Strobl
+ * @since 3.2
+ */
+public class BindableMongoExpression implements MongoExpression {
+
+	private final String expressionString;
+
+	@Nullable //
+	private final CodecRegistryProvider codecRegistryProvider;
+
+	@Nullable //
+	private final Object[] args;
+
+	private final Lazy<Document> target;
+
+	/**
+	 * Create a new instance of {@link BindableMongoExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @param args can be {@literal null}.
+	 */
+	public BindableMongoExpression(String expression, @Nullable Object[] args) {
+		this(expression, null, args);
+	}
+
+	/**
+	 * Create a new instance of {@link BindableMongoExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @param codecRegistryProvider can be {@literal null}.
+	 * @param args can be {@literal null}.
+	 */
+	public BindableMongoExpression(String expression, @Nullable CodecRegistryProvider codecRegistryProvider,
+			@Nullable Object[] args) {
+
+		this.expressionString = expression;
+		this.codecRegistryProvider = codecRegistryProvider;
+		this.args = args;
+		this.target = Lazy.of(this::parse);
+	}
+
+	/**
+	 * Provide the {@link CodecRegistry} used to convert expressions.
+	 *
+	 * @param codecRegistry must not be {@literal null}.
+	 * @return new instance of {@link BindableMongoExpression}.
+	 */
+	public BindableMongoExpression withCodecRegistry(CodecRegistry codecRegistry) {
+		return new BindableMongoExpression(expressionString, () -> codecRegistry, args);
+	}
+
+	/**
+	 * Provide the arguments to bind to the placeholders via their index.
+	 *
+	 * @param args must not be {@literal null}.
+	 * @return new instance of {@link BindableMongoExpression}.
+	 */
+	public BindableMongoExpression bind(Object... args) {
+		return new BindableMongoExpression(expressionString, codecRegistryProvider, args);
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.MongoExpression#toDocument()
+	 */
+	@Override
+	public Document toDocument() {
+		return target.get();
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see java.lang.Object#toString()
+	 */
+	@Override
+	public String toString() {
+		return "BindableMongoExpression{" + "expressionString='" + expressionString + '\'' + ", args=" + args + '}';
+	}
+
+	private String wrapJsonIfNecessary(String json) {
+
+		if (StringUtils.hasText(json) && (json.startsWith("{") && json.endsWith("}"))) {
+			return json;
+		}
+
+		return "{" + json + "}";
+	}
+
+	private Document parse() {
+
+		String expression = wrapJsonIfNecessary(expressionString);
+
+		if (ObjectUtils.isEmpty(args)) {
+
+			if (codecRegistryProvider == null) {
+				return Document.parse(expression);
+			}
+
+			return Document.parse(expression, codecRegistryProvider.getCodecFor(Document.class)
+					.orElseGet(() -> new DocumentCodec(codecRegistryProvider.getCodecRegistry())));
+		}
+
+		ParameterBindingDocumentCodec codec = codecRegistryProvider == null ? new ParameterBindingDocumentCodec()
+				: new ParameterBindingDocumentCodec(codecRegistryProvider.getCodecRegistry());
+		return codec.decode(expression, args);
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/MongoExpression.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2021 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;
+
+/**
+ * Wrapper object for MongoDB expressions like {@code $toUpper : $name} that manifest as {@link org.bson.Document} when
+ * passed on to the driver.
+ * <p />
+ * A set of predefined {@link MongoExpression expressions}, including a
+ * {@link org.springframework.data.mongodb.core.aggregation.AggregationSpELExpression SpEL based variant} for method
+ * like expressions (eg. {@code toUpper(name)}) are available via the
+ * {@link org.springframework.data.mongodb.core.aggregation Aggregation API}.
+ *
+ * @author Christoph Strobl
+ * @since 3.2
+ * @see org.springframework.data.mongodb.core.aggregation.ArithmeticOperators
+ * @see org.springframework.data.mongodb.core.aggregation.ArrayOperators
+ * @see org.springframework.data.mongodb.core.aggregation.ComparisonOperators
+ * @see org.springframework.data.mongodb.core.aggregation.ConditionalOperators
+ * @see org.springframework.data.mongodb.core.aggregation.ConvertOperators
+ * @see org.springframework.data.mongodb.core.aggregation.DateOperators
+ * @see org.springframework.data.mongodb.core.aggregation.ObjectOperators
+ * @see org.springframework.data.mongodb.core.aggregation.SetOperators
+ * @see org.springframework.data.mongodb.core.aggregation.StringOperators
+ */
+@FunctionalInterface
+public interface MongoExpression {
+
+	/**
+	 * Obtain the native {@link org.bson.Document} representation.
+	 *
+	 * @return never {@literal null}.
+	 */
+	org.bson.Document toDocument();
+
+	/**
+	 * Create a new {@link MongoExpression} from plain {@link String} (eg. {@code $toUpper : $name}). <br />
+	 * The given expression will be wrapped with <code>{ ... }</code> to match an actual MongoDB {@link org.bson.Document}
+	 * if necessary.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return new instance of {@link MongoExpression}.
+	 */
+	static MongoExpression create(String expression) {
+		return new BindableMongoExpression(expression, null);
+	}
+
+	/**
+	 * Create a new {@link MongoExpression} from plain {@link String} containing placeholders (eg. {@code $toUpper : ?0})
+	 * that will be resolved on first call of {@link #toDocument()}. <br />
+	 * The given expression will be wrapped with <code>{ ... }</code> to match an actual MongoDB {@link org.bson.Document}
+	 * if necessary.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return new instance of {@link MongoExpression}.
+	 */
+	static MongoExpression create(String expression, Object... args) {
+		return new BindableMongoExpression(expression, args);
+	}
+}

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

@@ -17,6 +17,7 @@
 
 import java.util.List;
 import java.util.Map;
+import java.util.Map.Entry;
 import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
@@ -31,8 +32,10 @@
 import org.springframework.data.mapping.PropertyReferenceException;
 import org.springframework.data.mapping.context.MappingContext;
 import org.springframework.data.mongodb.CodecRegistryProvider;
+import org.springframework.data.mongodb.MongoExpression;
 import org.springframework.data.mongodb.core.MappedDocument.MappedUpdate;
 import org.springframework.data.mongodb.core.aggregation.Aggregation;
+import org.springframework.data.mongodb.core.aggregation.AggregationExpression;
 import org.springframework.data.mongodb.core.aggregation.AggregationOperationContext;
 import org.springframework.data.mongodb.core.aggregation.AggregationOptions;
 import org.springframework.data.mongodb.core.aggregation.AggregationPipeline;
@@ -288,7 +291,21 @@ <T> Document getMappedQuery(@Nullable MongoPersistentEntity<T> entity) {
 		Document getMappedFields(@Nullable MongoPersistentEntity<?> entity, Class<?> targetType,
 				ProjectionFactory projectionFactory) {
 
-			Document fields = query.getFieldsObject();
+			Document fields = new Document();
+
+			for (Entry<String, Object> entry : query.getFieldsObject().entrySet()) {
+
+				if (entry.getValue() instanceof MongoExpression) {
+
+					AggregationOperationContext ctx = entity == null ? Aggregation.DEFAULT_CONTEXT
+							: new RelaxedTypeBasedAggregationOperationContext(entity.getType(), mappingContext, queryMapper);
+
+					fields.put(entry.getKey(), AggregationExpression.from((MongoExpression) entry.getValue()).toDocument(ctx));
+				} else {
+					fields.put(entry.getKey(), entry.getValue());
+				}
+			}
+
 			Document mappedFields = fields;
 
 			if (entity == null) {

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

@@ -16,6 +16,7 @@
 package org.springframework.data.mongodb.core.aggregation;
 
 import org.bson.Document;
+import org.springframework.data.mongodb.MongoExpression;
 
 /**
  * An {@link AggregationExpression} can be used with field expressions in aggregation pipeline stages like
@@ -25,7 +26,37 @@
  * @author Oliver Gierke
  * @author Christoph Strobl
  */
-public interface AggregationExpression {
+public interface AggregationExpression extends MongoExpression {
+
+	/**
+	 * Obtain the as is (unmapped) representation of the {@link AggregationExpression}. Use
+	 * {@link #toDocument(AggregationOperationContext)} with a matching {@link AggregationOperationContext context} to
+	 * engage domain type mapping including field name resolution.
+	 *
+	 * @see org.springframework.data.mongodb.MongoExpression#toDocument()
+	 */
+	@Override
+	default Document toDocument() {
+		return toDocument(Aggregation.DEFAULT_CONTEXT);
+	}
+
+	/**
+	 * Create an {@link AggregationExpression} out of a given {@link MongoExpression} to ensure the resulting
+	 * {@link MongoExpression#toDocument() Document} is mapped against the {@link AggregationOperationContext}. <br />
+	 * If the given expression is already an {@link AggregationExpression} the very same instance is returned.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 3.2
+	 */
+	static AggregationExpression from(MongoExpression expression) {
+
+		if (expression instanceof AggregationExpression) {
+			return AggregationExpression.class.cast(expression);
+		}
+
+		return (context) -> context.getMappedObject(expression.toDocument());
+	}
 
 	/**
 	 * Turns the {@link AggregationExpression} into a {@link Document} within the given