fix(GraphQL): fix lambda querying a lambda field in case of no data.

graphql/e2e/common/common.go

@@ -911,6 +911,7 @@ func RunAll(t *testing.T) {
 	t.Run("lambda on interface field", lambdaOnInterfaceField)
 	t.Run("lambda on query using dql", lambdaOnQueryUsingDql)
 	t.Run("lambda on mutation using graphql", lambdaOnMutationUsingGraphQL)
+	t.Run("lambda on query with no unique parents", lambdaOnQueryWithNoUniqueParents)
 	t.Run("query lambda field in a mutation with duplicate @id", lambdaInMutationWithDuplicateId)
 	t.Run("lambda with apollo federation", lambdaWithApolloFederation)
 }

graphql/e2e/common/lambda.go

@@ -193,6 +193,24 @@ func lambdaOnMutationUsingGraphQL(t *testing.T) {
 	deleteAuthors(t, []string{addResp.AuthorID}, nil)
 }
 
+func lambdaOnQueryWithNoUniqueParents(t *testing.T) {
+	queryBookParams := &GraphQLParams{Query: `
+	query{
+		getBook(bookId: 1){
+		  name
+		  desc
+		  summary
+		}
+	  }
+	`}
+
+	resp := queryBookParams.ExecuteAsPost(t, GraphqlURL)
+	RequireNoGQLErrors(t, resp)
+	testutil.CompareJSON(t, `{
+		"getBook": null
+	}`, string(resp.Data))
+}
+
 // See: https://discuss.dgraph.io/t/slash-graphql-lambda-bug/12233
 func lambdaInMutationWithDuplicateId(t *testing.T) {
 	addStudentParams := &GraphQLParams{Query: `

query/outputnode_graphql.go

@@ -638,7 +638,7 @@ func (genc *graphQLEncoder) processCustomFields(field gqlSchema.Field, n fastJso
 			// * a linear search to find the correct fastJson node for a custom field, or
 			// * first fix the order of custom fastJson nodes and then continue the encoding, or
 			// * create a map from custom fastJson node attr to the custom fastJson node,
-			//   so that whenever a custom field in encountered in the selection set,
+			//   so that whenever a custom field is encountered in the selection set,
 			//   just use the map to find out the fastJson node for that field.
 			// The last option seems better.
 
@@ -836,6 +836,10 @@ func (genc *graphQLEncoder) resolveCustomField(childField gqlSchema.Field,
 		}
 	}
 
+	if len(uniqueParents) == 0 {
+		return
+	}
+
 	switch fconf.Mode {
 	case gqlSchema.SINGLE:
 		// In SINGLE mode, we can consider steps 2-5 as a single isolated unit of computation,