DATAMONGO-1734 - Polish MongoTemplate.exists execution.

Optimize execution by using count() limited to 1 element.

Original pull request: #479.

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

@@ -20,6 +20,7 @@ import static org.springframework.data.mongodb.core.query.SerializationUtils.*;
 
 import lombok.AccessLevel;
 import lombok.AllArgsConstructor;
+import lombok.RequiredArgsConstructor;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -131,6 +132,7 @@ import com.mongodb.client.MapReduceIterable;
 import com.mongodb.client.MongoCollection;
 import com.mongodb.client.MongoCursor;
 import com.mongodb.client.MongoDatabase;
+import com.mongodb.client.model.CountOptions;
 import com.mongodb.client.model.CreateCollectionOptions;
 import com.mongodb.client.model.DeleteOptions;
 import com.mongodb.client.model.Filters;
@@ -609,14 +611,9 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		}
 
 		Document mappedQuery = queryMapper.getMappedObject(query.getQueryObject(), getPersistentEntity(entityClass));
-		FindIterable<Document> iterable = execute(collectionName, new FindCallback(mappedQuery));
 
-		if (query.getCollation().isPresent()) {
+		return execute(collectionName, new ExistsCallback(mappedQuery,
-			iterable = iterable
+				query.getCollation().map(org.springframework.data.mongodb.core.Collation::toMongoCollation).orElse(null)));
-					.collation(query.getCollation().map(org.springframework.data.mongodb.core.Collation::toMongoCollation).get());
-		}
-
-		return iterable.iterator().hasNext();
 	}
 
 	// Find methods that take a Query to express the query and that return a List of objects.
@@ -2429,6 +2426,25 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		}
 	}
 
+	/**
+	 * Optimized {@link CollectionCallback} that takes an already mappend query and a nullable
+	 * {@link com.mongodb.client.model.Collation} to execute a count query limited to one element.
+	 *
+	 * @author Christoph Strobl
+	 * @since 2.0
+	 */
+	@RequiredArgsConstructor
+	private static class ExistsCallback implements CollectionCallback<Boolean> {
+
+		private final Document mappedQuery;
+		private final com.mongodb.client.model.Collation collation;
+
+		@Override
+		public Boolean doInCollection(MongoCollection<Document> collection) throws MongoException, DataAccessException {
+			return collection.count(mappedQuery, new CountOptions().limit(1).collation(collation)) > 0;
+		}
+	}
+
 	/**
 	 * Simple {@link CollectionCallback} that takes a query {@link Document} plus an optional fields specification
 	 * {@link Document} and executes that against the {@link DBCollection}.

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

@@ -81,6 +81,7 @@ import com.mongodb.client.MapReduceIterable;
 import com.mongodb.client.MongoCollection;
 import com.mongodb.client.MongoCursor;
 import com.mongodb.client.MongoDatabase;
+import com.mongodb.client.model.CountOptions;
 import com.mongodb.client.model.DeleteOptions;
 import com.mongodb.client.model.FindOneAndDeleteOptions;
 import com.mongodb.client.model.FindOneAndUpdateOptions;
@@ -123,6 +124,7 @@ public class MongoTemplateUnitTests extends MongoOperationsUnitTests {
 		when(db.runCommand(Mockito.any(), Mockito.any(Class.class))).thenReturn(commandResultDocument);
 		when(collection.find(Mockito.any(org.bson.Document.class))).thenReturn(findIterable);
 		when(collection.mapReduce(Mockito.any(), Mockito.any())).thenReturn(mapReduceIterable);
+		when(collection.count(any(), any())).thenReturn(1L);
 		when(findIterable.projection(Mockito.any())).thenReturn(findIterable);
 		when(findIterable.sort(Mockito.any(org.bson.Document.class))).thenReturn(findIterable);
 		when(findIterable.modifiers(Mockito.any(org.bson.Document.class))).thenReturn(findIterable);
@@ -666,7 +668,11 @@ public class MongoTemplateUnitTests extends MongoOperationsUnitTests {
 
 		template.exists(new BasicQuery("{}").collation(Collation.of("fr")), AutogenerateableId.class);
 
-		verify(findIterable).collation(eq(com.mongodb.client.model.Collation.builder().locale("fr").build()));
+		ArgumentCaptor<CountOptions> options = ArgumentCaptor.forClass(CountOptions.class);
+		verify(collection).count(any(), options.capture());
+
+		assertThat(options.getValue().getCollation(),
+				is(equalTo(com.mongodb.client.model.Collation.builder().locale("fr").build())));
 	}
 
 	@Test // DATAMONGO-1518