Polishing.

Reorder methods. Tweak Javadoc and documentation wording. Mention projection expressions in the what's new section. Reformat code.

See #3583
Original pull request: #3585.

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

@@ -15,6 +15,8 @@
  */
 package org.springframework.data.mongodb;
 
+import java.util.Arrays;
+
 import org.bson.Document;
 import org.bson.codecs.DocumentCodec;
 import org.bson.codecs.configuration.CodecRegistry;
@@ -49,11 +51,9 @@ public class BindableMongoExpression implements MongoExpression {
 
 	private final String expressionString;
 
-	@Nullable //
+	private final @Nullable CodecRegistryProvider codecRegistryProvider;
-	private final CodecRegistryProvider codecRegistryProvider;
 
-	@Nullable //
+	private final @Nullable Object[] args;
-	private final Object[] args;
 
 	private final Lazy<Document> target;
 
@@ -118,16 +118,8 @@ public class BindableMongoExpression implements MongoExpression {
 	 */
 	@Override
 	public String toString() {
-		return "BindableMongoExpression{" + "expressionString='" + expressionString + '\'' + ", args=" + args + '}';
+		return "BindableMongoExpression{" + "expressionString='" + expressionString + '\'' + ", args="
-	}
+				+ Arrays.toString(args) + '}';
-
-	private String wrapJsonIfNecessary(String json) {
-
-		if (StringUtils.hasText(json) && (json.startsWith("{") && json.endsWith("}"))) {
-			return json;
-		}
-
-		return "{" + json + "}";
 	}
 
 	private Document parse() {
@@ -148,4 +140,13 @@ public class BindableMongoExpression implements MongoExpression {
 				: new ParameterBindingDocumentCodec(codecRegistryProvider.getCodecRegistry());
 		return codec.decode(expression, args);
 	}
+
+	private static String wrapJsonIfNecessary(String json) {
+
+		if (StringUtils.hasText(json) && (json.startsWith("{") && json.endsWith("}"))) {
+			return json;
+		}
+
+		return "{" + json + "}";
+	}
 }

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

@@ -39,13 +39,6 @@ package org.springframework.data.mongodb;
 @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}
@@ -70,4 +63,11 @@ public interface MongoExpression {
 	static MongoExpression create(String expression, Object... args) {
 		return new BindableMongoExpression(expression, args);
 	}
+
+	/**
+	 * Obtain the native {@link org.bson.Document} representation.
+	 *
+	 * @return never {@literal null}.
+	 */
+	org.bson.Document toDocument();
 }

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

@@ -28,18 +28,6 @@ import org.springframework.data.mongodb.MongoExpression;
  */
 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 />
@@ -58,6 +46,18 @@ public interface AggregationExpression extends MongoExpression {
 		return (context) -> context.getMappedObject(expression.toDocument());
 	}
 
+	/**
+	 * 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);
+	}
+
 	/**
 	 * Turns the {@link AggregationExpression} into a {@link Document} within the given
 	 * {@link AggregationOperationContext}.

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

@@ -77,7 +77,7 @@ public class Field {
 	 * </pre>
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return new instance of {@link FieldProjectionExpression} - you still need to define the target field name via
+	 * @return new instance of {@link FieldProjectionExpression}. Define the target field name through
 	 *         {@link FieldProjectionExpression#as(String) as(String)}.
 	 * @since 3.2
 	 */
@@ -284,7 +284,7 @@ public class Field {
 	public interface FieldProjectionExpression {
 
 		/**
-		 * Set the name to be used in the result.
+		 * Set the name to be used in the result and return a {@link Field}.
 		 *
 		 * @param name must not be {@literal null}.
 		 * @return the calling instance {@link Field}.

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

@@ -39,6 +39,8 @@ import org.springframework.data.mongodb.test.util.MongoTestTemplate;
 import org.springframework.data.mongodb.test.util.Template;
 
 /**
+ * Integration tests for {@link org.springframework.data.mongodb.core.query.Field}.
+ *
  * @author Christoph Strobl
  */
 @ExtendWith(MongoTemplateExtension.class)

src/main/asciidoc/new-features.adoc

@@ -5,6 +5,7 @@
 == What's New in Spring Data MongoDB 3.2
 
 * Support for <<embedded-entities,Embedded Types>> to unwrap nested objects into the parent `Document`.
+* <<mongo-template.querying.field-selection,Support expressions to define field projections>>.
 
 [[new-features.3.1]]
 == What's New in Spring Data MongoDB 3.1

src/main/asciidoc/reference/mongodb.adoc

@@ -1251,7 +1251,7 @@ The `Query` class has some additional methods that provide options for the query
 ==== Selecting fields
 
 MongoDB supports https://docs.mongodb.com/manual/tutorial/project-fields-from-query-results/[projecting fields] returned by a query.
-A projection can in- & exclude fields (the `_id` field is always included unless explicitly excluded) based on their name.
+A projection can include and exclude fields (the `_id` field is always included unless explicitly excluded) based on their name.
 
 .Selecting result fields
 ====
@@ -1284,9 +1284,9 @@ query.fields().include("address.city") <4>
 <4> Result will contain the `_id` and and `address` object that only contains the `city` field via `{ "address.city" : 1 }`.
 ====
 
-Starting with MongoDB 4.4 it is possible to use the aggregation expressions syntax for field projections as shown below.
+Starting with MongoDB 4.4 you can use aggregation expressions for field projections as shown below:
 
-.Computing result fields with expressions
+.Computing result fields using expressions
 ====
 [source,java]
 ----
@@ -1302,13 +1302,13 @@ query.fields()
   .project(AggregationSpELExpression.expressionOf("toUpper(lastname)")) <4>
   .as("last_name");
 ----
-<1> Use a native expression. The used field names must refer to the ones of the document within the database.
+<1> Use a native expression. The used field name must refer to field names within the database document.
-<2> Assign the field name that shall hold the expression result in the target document. The resulting field name will never be mapped against the domain model.
+<2> Assign the field name to which the expression result is projected. The resulting field name is not mapped against the domain model.
 <3> Use an `AggregationExpression`. Other than native `MongoExpression`, field names are mapped to the ones used in the domain model.
 <4> Use SpEL along with an `AggregationExpression` to invoke expression functions. Field names are mapped to the ones used in the domain model.
 ====
 
-`@Query(fields='...')` allows usage of expression field projections at `Repository` level as described in <<mongodb.repositories.queries.json-based>>.
+`@Query(fields="…")` allows usage of expression field projections at `Repository` level as described in <<mongodb.repositories.queries.json-based>>.
 
 [[mongo-template.querying]]
 === Methods for Querying for Documents