fix: Panic with filter on unique composite index on relation (sourcen…

…etwork#3020)

## Relevant issue(s)

Resolves sourcenetwork#3016

## Description

Fix panic with filter on unique composite index on relation.

internal/db/fetcher/indexer_iterators.go

@@ -141,6 +141,9 @@ func (iter *indexPrefixIterator) Next() (indexIterResult, error) {
 }
 
 func (iter *indexPrefixIterator) Close() error {
+	if iter.resultIter == nil {
+		return nil
+	}
 	return iter.resultIter.Close()
 }
 

tests/integration/index/query_with_relation_filter_test.go

@@ -1018,36 +1018,36 @@ func TestQueryWithIndexOnManyToOne_MultipleViaOneToMany(t *testing.T) {
 	testUtils.ExecuteTestCase(t, test)
 }
 
-func TestQueryWithIndex_UniqueIndexOnChildWithEmptyParentCollection(t *testing.T) {
+func TestQueryWithUniqueIndex_WithFilterOnChildIndexedField_ShouldFetch(t *testing.T) {
 	test := testUtils.TestCase{
 		Actions: []any{
 			testUtils.SchemaUpdate{
 				Schema: `
-					type Action {
-						key: String @index(unique: true)
-						playerActions: [PlayerAction]
+					type User {
+						name: String @index(unique: true)
+						devices: [Device]
 					}
 
-					type PlayerAction {
-						deleted: Boolean
-						action: Action
+					type Device {
+						trusted: Boolean
+						owner: User
 					}
 				`,
 			},
 			testUtils.CreateDoc{
 				CollectionID: 0,
 				DocMap: map[string]any{
-					"key": "ACTION_KEY",
+					"name": "John",
 				},
 			},
 			testUtils.Request{
 				Request: `query {
-					PlayerAction(filter: {action: {key: {_eq: "ACTION_KEY"}}}) {
-						deleted
+					Device(filter: {owner: {name: {_eq: "John"}}}) {
+						trusted
 					}
 				}`,
 				Results: map[string]any{
-					"PlayerAction": []map[string]any{},
+					"Device": []map[string]any{},
 				},
 			},
 		},

tests/integration/index/query_with_unique_index_on_relation_filter_test.go

@@ -0,0 +1,72 @@
+// Copyright 2024 Democratized Data Foundation
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+package index
+
+import (
+	"testing"
+
+	testUtils "github.com/sourcenetwork/defradb/tests/integration"
+)
+
+func TestQueryWithUniqueCompositeIndex_WithFilterOnIndexedRelation_ShouldFilter(t *testing.T) {
+	test := testUtils.TestCase{
+		Actions: []any{
+			testUtils.SchemaUpdate{
+				Schema: `
+					type User {
+						name: String 
+						devices: [Device]
+					}
+
+					type Device  {
+						manufacturer: String 
+						owner: User @index(unique: true, includes: [{name: "manufacturer"}])
+					}
+				`,
+			},
+			testUtils.CreateDoc{
+				CollectionID: 0,
+				DocMap: map[string]any{
+					"name": "John",
+				},
+			},
+			testUtils.CreateDoc{
+				CollectionID: 1,
+				DocMap: map[string]any{
+					"manufacturer": "Apple",
+					"owner_id":     testUtils.NewDocIndex(0, 0),
+				},
+			},
+			testUtils.Request{
+				Request: `query {
+					User {
+						name
+						devices(filter: {owner_id: {_eq: "bae-1ef746f8-821e-586f-99b2-4cb1fb9b782f"}}) {
+							manufacturer
+						}
+					}
+				}`,
+				Results: map[string]any{
+					"User": []map[string]any{
+						{
+							"name": "John",
+							"devices": []map[string]any{
+								{"manufacturer": "Apple"},
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	testUtils.ExecuteTestCase(t, test)
+}