Improve DataFetcher handling for connection field

The ConnectionDataFetcher now handles a null return value from the delegate DataFetcher, turning leniently into an empty connection. Also log a debug message in case of a TrivialDataFetcher mapped to a connection field, and do not decorate it to begin with. Closes gh-707

Author: rstoyanchev

spring-graphql/src/main/java/org/springframework/graphql/data/pagination/ConnectionFieldTypeVisitor.java

@@ -22,6 +22,7 @@
 import java.util.List;
 import java.util.concurrent.CompletionStage;
 
+import graphql.TrivialDataFetcher;
 import graphql.relay.Connection;
 import graphql.relay.DefaultConnection;
 import graphql.relay.DefaultConnectionCursor;
@@ -40,8 +41,11 @@
 import graphql.schema.GraphQLTypeVisitorStub;
 import graphql.util.TraversalControl;
 import graphql.util.TraverserContext;
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 import reactor.core.publisher.Mono;
 
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
@@ -57,6 +61,9 @@
  */
 public final class ConnectionFieldTypeVisitor extends GraphQLTypeVisitorStub {
 
+private static Log logger = LogFactory.getLog(ConnectionFieldTypeVisitor.class);
+
+
 private final ConnectionAdapter adapter;
 
 
@@ -79,7 +86,16 @@ public TraversalControl visitGraphQLFieldDefinition(
 }
 
 if (isConnectionField(fieldDefinition)) {
-codeRegistry.dataFetcher(parent, fieldDefinition, new ConnectionDataFetcher(dataFetcher, adapter));
+if (dataFetcher instanceof TrivialDataFetcher<?>) {
+if (logger.isDebugEnabled()) {
+logger.debug("Connection field " +
+"'" + parent.getName() + ":" + fieldDefinition.getName() + "' " +
+"is mapped to trivial data fetcher: " + dataFetcher.getClass().getName());
+}
+}
+else {
+codeRegistry.dataFetcher(parent, fieldDefinition, new ConnectionDataFetcher(dataFetcher, adapter));
+}
 }
 
 return TraversalControl.CONTINUE;
@@ -138,12 +154,14 @@ else if (result instanceof CompletionStage<?> stage) {
 }
 
 @SuppressWarnings("unchecked")
-private <T> Connection<T> adapt(Object container) {
+private <T> Connection<T> adapt(@Nullable Object container) {
 if (container instanceof Connection<?> connection) {
 return (Connection<T>) connection;
 }
 
-Collection<T> nodes = this.adapter.getContent(container);
+Collection<T> nodes = (container != null ?
+this.adapter.getContent(container) : Collections.emptyList());
+
 if (nodes.isEmpty()) {
 return (Connection<T>) EMPTY_CONNECTION;
 }

spring-graphql/src/test/java/org/springframework/graphql/data/pagination/ConnectionFieldTypeVisitorTests.java

@@ -19,6 +19,7 @@
 import java.util.Collection;
 import java.util.List;
 
+import graphql.schema.PropertyDataFetcher;
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Mono;
 
@@ -38,7 +39,7 @@ public class ConnectionFieldTypeVisitorTests {
 
 
 @Test
-void dataFetcherDecoration() {
+void paginationDataFetcher() {
 
 String document = BookSource.booksConnectionQuery("");
 
@@ -73,6 +74,49 @@ void dataFetcherDecoration() {
 );
 }
 
+@Test // gh-707
+void trivialDataFetcherIsSkipped() {
+
+TestConnectionAdapter adapter = new TestConnectionAdapter();
+adapter.setInitialOffset(30);
+adapter.setHasNext(true);
+
+Mono<ExecutionGraphQlResponse> response = GraphQlSetup.schemaResource(BookSource.paginationSchema)
+.dataFetcher("Query", "books", new PropertyDataFetcher<>("books"))
+.typeDefinitionConfigurer(new ConnectionTypeDefinitionConfigurer())
+.typeVisitor(ConnectionFieldTypeVisitor.create(List.of(adapter)))
+.toGraphQlService()
+.execute(TestExecutionRequest.forDocument(BookSource.booksConnectionQuery("")));
+
+ResponseHelper.forResponse(response).assertData("{\"books\":null}");
+}
+
+@Test // gh-707
+void nullValueTreatedAsEmptyConnection() {
+
+TestConnectionAdapter adapter = new TestConnectionAdapter();
+adapter.setInitialOffset(30);
+adapter.setHasNext(true);
+
+Mono<ExecutionGraphQlResponse> response = GraphQlSetup.schemaResource(BookSource.paginationSchema)
+.dataFetcher("Query", "books", environment -> null)
+.typeDefinitionConfigurer(new ConnectionTypeDefinitionConfigurer())
+.typeVisitor(ConnectionFieldTypeVisitor.create(List.of(adapter)))
+.toGraphQlService()
+.execute(TestExecutionRequest.forDocument(BookSource.booksConnectionQuery("")));
+
+ResponseHelper.forResponse(response).assertData(
+"{\"books\":{" +
+"\"edges\":[]," +
+"\"pageInfo\":{" +
+"\"startCursor\":null," +
+"\"endCursor\":null," +
+"\"hasPreviousPage\":false," +
+"\"hasNextPage\":false}" +
+"}}"
+);
+}
+
 
 private static class TestConnectionAdapter implements ConnectionAdapter {