Fixed tree grid records' order after hierarchizing. (#16329)

* fix(TreeGrid): Preserve the original order in the collection when hierarchizing + a unit test.

Author: gedinakova

projects/igniteui-angular/src/lib/grids/tree-grid/tree-grid-integration.spec.ts

@@ -198,6 +198,73 @@ describe('IgxTreeGrid - Integration #tGrid', () => {
             treeGrid = fix.componentInstance.treeGrid;
         });
 
+        it('should preserve the order of records on inner levels', () => {
+            fix = TestBed.createComponent(IgxTreeGridPrimaryForeignKeyComponent);
+            fix.componentInstance.sortByName = true;
+            fix.detectChanges();
+            treeGrid = fix.componentInstance.treeGrid;
+
+            const expectedFlatData = [
+                {
+                    "ID": 1,
+                    "ParentID": -1,
+                    "Name": "Casey Houston",
+                    "JobTitle": "Vice President",
+                    "Age": 32
+                },
+                {
+                    "ID": 2,
+                    "ParentID": 1,
+                    "Name": "Gilberto Todd",
+                    "JobTitle": "Director",
+                    "Age": 41
+                },
+                {
+                    "ID": 7,
+                    "ParentID": 2,
+                    "Name": "Debra Morton",
+                    "JobTitle": "Associate Software Developer",
+                    "Age": 35
+                },
+                {
+                    "ID": 3,
+                    "ParentID": 2,
+                    "Name": "Tanya Bennett",
+                    "JobTitle": "Director",
+                    "Age": 29
+                },
+                {
+                    "ID": 4,
+                    "ParentID": 1,
+                    "Name": "Jack Simon",
+                    "JobTitle": "Software Developer",
+                    "Age": 33
+                },
+                {
+                    "ID": 10,
+                    "ParentID": -1,
+                    "Name": "Eduardo Ramirez",
+                    "JobTitle": "Manager",
+                    "Age": 53
+                },
+                {
+                    "ID": 9,
+                    "ParentID": 10,
+                    "Name": "Leslie Hansen",
+                    "JobTitle": "Associate Software Developer",
+                    "Age": 44
+                },
+                {
+                    "ID": 6,
+                    "ParentID": -1,
+                    "Name": "Erma Walsh",
+                    "JobTitle": "CEO",
+                    "Age": 52
+                },
+            ]
+            expect(treeGrid.flatData).toEqual(expectedFlatData);
+        });
+
         it('should transform a non-tree column into a tree column when pinning it', () => {
             TreeGridFunctions.verifyTreeColumn(fix, 'ID', 5);
 

projects/igniteui-angular/src/lib/grids/tree-grid/tree-grid.pipes.ts

@@ -53,30 +53,44 @@ export class IgxTreeGridHierarchizingPipe implements PipeTransform {
         return primaryKey ? rowData[primaryKey] : rowData;
     }
 
-    private hierarchizeFlatData(collection: any[], primaryKey: string, foreignKey: string,
-        map: Map<any, ITreeGridRecord>, flatData: any[]):
-        ITreeGridRecord[] {
-        const result: ITreeGridRecord[] = [];
-        const missingParentRecords: ITreeGridRecord[] = [];
+    /**
+     * Converts a flat array of data into a hierarchical (tree) structure,
+     * preserving the original order of the records among siblings.
+     *
+     * It uses a two-pass approach:
+     * 1. Creates all ITreeGridRecord objects and populates the Map for quick lookup.
+     * 2. Links the records by iterating again, ensuring children are added to
+     * their parent's children array in the order they appeared in the
+     * original collection.
+     *
+     * @param collection The flat array of data to be hierarchized. This is the array whose order should be preserved.
+     * @param primaryKey The name of the property in the data objects that serves as the unique identifier (e.g., 'id').
+     * @param foreignKey The name of the property in the data objects that links to the parent's primary key (e.g., 'parentId').
+     * @param map A pre-existing Map object (key: primaryKey value, value: ITreeGridRecord) used to store and quickly look up all created records.
+     * @param flatData The original flat data array. Used for passing to the setIndentationLevels method (not directly used for hierarchy building).
+     * @returns An array of ITreeGridRecord objects representing the root nodes of the hierarchy, ordered as they appeared in the original collection.
+     */
+    private hierarchizeFlatData(
+        collection: any[],
+        primaryKey: string,
+        foreignKey: string,
+        map: Map<any, ITreeGridRecord>,
+        flatData: any[]
+    ): ITreeGridRecord[] {
         collection.forEach(row => {
             const record: ITreeGridRecord = {
                 key: this.getRowID(primaryKey, row),
                 data: row,
                 children: []
             };
-            const parent = map.get(row[foreignKey]);
-            if (parent) {
-                record.parent = parent;
-                parent.children.push(record);
-            } else {
-                missingParentRecords.push(record);
-            }
-
             map.set(row[primaryKey], record);
         });
 
-        missingParentRecords.forEach(record => {
-            const parent = map.get(record.data[foreignKey]);
+        const result: ITreeGridRecord[] = [];
+        collection.forEach(row => {
+            const record: ITreeGridRecord = map.get(row[primaryKey])!;
+            const parent = map.get(row[foreignKey]);
+
             if (parent) {
                 record.parent = parent;
                 parent.children.push(record);

projects/igniteui-angular/src/lib/test-utils/tree-grid-components.spec.ts

@@ -159,10 +159,17 @@ export class IgxTreeGridWithNoScrollsComponent {
     `,
     imports: [IgxTreeGridComponent, IgxColumnComponent, IgxPaginatorComponent]
 })
-export class IgxTreeGridPrimaryForeignKeyComponent {
+export class IgxTreeGridPrimaryForeignKeyComponent implements OnInit {
     @ViewChild(IgxTreeGridComponent, { static: true }) public treeGrid: IgxTreeGridComponent;
-    public data = SampleTestData.employeePrimaryForeignKeyTreeData();
+    public data = [];
     public paging = false;
+    public sortByName = false;
+
+    public ngOnInit(): void {
+        this.data = !this.sortByName
+                        ? SampleTestData.employeePrimaryForeignKeyTreeData()
+                        : SampleTestData.employeePrimaryForeignKeyTreeData().sort((a, b) => a.Name.localeCompare(b.Name));
+    }
 }
 
 @Component({