Add hidden support for index.

Closes: #4348
Original Pull Request: #4349

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

@@ -119,6 +119,10 @@ abstract class IndexConverters {
 				ops.wildcardProjection(indexOptions.get("wildcardProjection", Document.class));
 			}
 
+			if (indexOptions.containsKey("hidden")) {
+				ops = ops.hidden((Boolean) indexOptions.get("hidden"));
+			}
+
 			return ops;
 		};
 	}

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

@@ -42,6 +42,7 @@ public class Index implements IndexDefinition {
 	private boolean unique = false;
 	private boolean sparse = false;
 	private boolean background = false;
+	private boolean hidden = false;
 	private long expire = -1;
 	private Optional<IndexFilter> filter = Optional.empty();
 	private Optional<Collation> collation = Optional.empty();
@@ -98,6 +99,18 @@ public class Index implements IndexDefinition {
 		return this;
 	}
 
+	/**
+	 * Hidden indexes are not visible to the query planner and cannot be used to support a query.
+	 *
+	 * @return this.
+	 * @see <a href=
+	 *      "https://www.mongodb.com/docs/manual/core/index-hidden/">https://www.mongodb.com/docs/manual/core/index-hidden/</a>
+	 */
+	public Index hidden() {
+		this.hidden = true;
+		return this;
+	}
+
 	/**
 	 * Specifies TTL in seconds.
 	 *
@@ -195,6 +208,9 @@ public class Index implements IndexDefinition {
 		if (background) {
 			document.put("background", true);
 		}
+		if (hidden) {
+			document.put("hidden", true);
+		}
 		if (expire >= 0) {
 			document.put("expireAfterSeconds", expire);
 		}

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

@@ -52,6 +52,7 @@ public class IndexInfo {
 	private final boolean unique;
 	private final boolean sparse;
 	private final String language;
+	private final boolean hidden;
 	private @Nullable Duration expireAfter;
 	private @Nullable String partialFilterExpression;
 	private @Nullable Document collation;
@@ -64,6 +65,17 @@ public class IndexInfo {
 		this.unique = unique;
 		this.sparse = sparse;
 		this.language = language;
+		this.hidden = false;
+	}
+
+	public IndexInfo(List<IndexField> indexFields, String name, boolean unique, boolean sparse, String language, boolean hidden) {
+
+		this.indexFields = Collections.unmodifiableList(indexFields);
+		this.name = name;
+		this.unique = unique;
+		this.sparse = sparse;
+		this.language = language;
+		this.hidden = hidden;
 	}
 
 	/**
@@ -138,6 +150,8 @@ public class IndexInfo {
 			info.wildcardProjection = sourceDocument.get("wildcardProjection", Document.class);
 		}
 
+		boolean hidden = sourceDocument.containsKey("hidden") ? (Boolean) sourceDocument.get("hidden") : false;
+
 		return info;
 	}
 
@@ -259,12 +273,17 @@ public class IndexInfo {
 		return getIndexFields().stream().anyMatch(IndexField::isWildcard);
 	}
 
+	public boolean isHidden() {
+		return hidden;
+	}
+
+
 	@Override
 	public String toString() {
 
 		return "IndexInfo [indexFields=" + indexFields + ", name=" + name + ", unique=" + unique + ", sparse=" + sparse
 				+ ", language=" + language + ", partialFilterExpression=" + partialFilterExpression + ", collation=" + collation
-				+ ", expireAfterSeconds=" + ObjectUtils.nullSafeToString(expireAfter) + "]";
+				+ ", expireAfterSeconds=" + ObjectUtils.nullSafeToString(expireAfter) + ", hidden=" + hidden + "]";
 	}
 
 	@Override
@@ -279,6 +298,7 @@ public class IndexInfo {
 		result += 31 * ObjectUtils.nullSafeHashCode(partialFilterExpression);
 		result += 31 * ObjectUtils.nullSafeHashCode(collation);
 		result += 31 * ObjectUtils.nullSafeHashCode(expireAfter);
+		result += 31 * ObjectUtils.nullSafeHashCode(hidden);
 		return result;
 	}
 
@@ -326,6 +346,9 @@ public class IndexInfo {
 		if (!ObjectUtils.nullSafeEquals(expireAfter, other.expireAfter)) {
 			return false;
 		}
+		if (hidden != other.hidden) {
+			return false;
+		}
 		return true;
 	}
 

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

@@ -329,7 +329,7 @@ public class MongoTemplateTests {
 		p2.setAge(40);
 		template.insert(p2);
 
-		template.indexOps(Person.class).ensureIndex(new Index().on("age", Direction.DESC).unique());
+		template.indexOps(Person.class).ensureIndex(new Index().on("age", Direction.DESC).unique().hidden());
 
 		MongoCollection<org.bson.Document> coll = template.getCollection(template.getCollectionName(Person.class));
 		List<org.bson.Document> indexInfo = new ArrayList<>();
@@ -355,6 +355,7 @@ public class MongoTemplateTests {
 		IndexInfo ii = indexInfoList.get(1);
 		assertThat(ii.isUnique()).isTrue();
 		assertThat(ii.isSparse()).isFalse();
+		assertThat(ii.isHidden()).isTrue();
 
 		List<IndexField> indexFields = ii.getIndexFields();
 		IndexField field = indexFields.get(0);