如何将项目列表转换为json父子层次结构？

import json

def convert_to_json_hierarchy(projects):
    json_hierarchy = {}
    
    for project in projects:
        project_id = project['id']
        parent_id = project['parent_id']
        
        if parent_id is None:
            # 根项目
            json_hierarchy[project_id] = project
        else:
            # 子项目
            parent_project = json_hierarchy.get(parent_id, {})
            parent_project.setdefault('children', []).append(project)
            json_hierarchy[parent_id] = parent_project
    
    return json.dumps(list(json_hierarchy.values()))

# 示例项目列表
projects = [
    {'id': 1, 'name': '项目A', 'parent_id': None},
    {'id': 2, 'name': '项目B', 'parent_id': None},
    {'id': 3, 'name': '子项目A1', 'parent_id': 1},
    {'id': 4, 'name': '子项目A2', 'parent_id': 1},
    {'id': 5, 'name': '子项目B1', 'parent_id': 2},
    {'id': 6, 'name': '子项目B2', 'parent_id': 2},
]

# 转换为JSON父子层次结构
json_hierarchy = convert_to_json_hierarchy(projects)
print(json_hierarchy)

[
    {
        "id": 1,
        "name": "项目A",
        "parent_id": null,
        "children": [
            {
                "id": 3,
                "name": "子项目A1",
                "parent_id": 1
            },
            {
                "id": 4,
                "name": "子项目A2",
                "parent_id": 1
            }
        ]
    },
    {
        "id": 2,
        "name": "项目B",
        "parent_id": null,
        "children": [
            {
                "id": 5,
                "name": "子项目B1",
                "parent_id": 2
            },
            {
                "id": 6,
                "name": "子项目B2",
                "parent_id": 2
            }
        ]
    }
]