marshmallow之schema嵌套

schema嵌套

schema可以嵌套使用以表示对象间的关系(如外键关系)。

例如下例中Blog有一个用User对象表示的author属性：

import datetime as dt

class User(object):
    def __init__(self, name, email):
        self.name = name
        self.email = email
        self.created_at = dt.datetime.now()
        self.friends = []
        self.employer = None

class Blog(object):
    def __init__(self, title, author):
        self.title = title
        self.author = author  # A User object

使用Nested子类接收嵌套的schema表示二者的关系：

from marshmallow import Schema, fields, pprint

class UserSchema(Schema):
    name = fields.String()
    email = fields.Email()
    created_at = fields.DateTime()

class BlogSchema(Schema):
    title = fields.String()
    author = fields.Nested(UserSchema)

序列化后的blog对象将包含嵌套的user对象：

user = User(name="Monty", email="[e-mail]")
blog = Blog(title="Something Completely Different", author=user)
result = BlogSchema().dump(blog)
pprint(result)
# {'title': u'Something Completely Different',
#  'author': {'name': u'Monty',
#             'email': u'[e-mail]',
#             'created_at': '2014-08-17T14:58:57.600623+00:00'}}

如果field嵌套对象是一个集合，必须设置many=True，如collaborators = fields.Nested(UserSchema, many=True)

指定嵌套对象的序列化字段

设置only参数显式地指定对嵌套对象的哪些属性进行序列化：

class BlogSchema2(Schema):
    title = fields.String()
    author = fields.Nested(UserSchema, only=["email"])

schema = BlogSchema2()
result = schema.dump(blog)
pprint(result)
# {
#     'title': u'Something Completely Different',
#     'author': {'email': u'[e-mail]'}
# }

使用点分隔符可以表示深层嵌套对象的属性：

class SiteSchema(Schema):
    blog = fields.Nested(BlogSchema2)

schema = SiteSchema(only=['blog.author.email'])
result, errors = schema.dump(site)
pprint(result)
# {
#     'blog': {
#         'author': {'email': u'[e-mail]'}
#     }
# }

如果给only参数传递的是字符串(上面的例子传递的是列表)，将返回单个值(上面的例子返回的是键值映射)或值的列表(需要设置many=True)：

class UserSchema(Schema):
    name = fields.String()
    email = fields.Email()
    friends = fields.Nested('self', only='name', many=True)
# ... create ``user`` ...
result, errors = UserSchema().dump(user)
pprint(result)
# {
#     "name": "Steve",
#     "email": "[e-mail]",
#     "friends": ["Mike", "Joe"]
# }

双向嵌套

对于两个互相嵌套的对象，可以使用类名引用嵌套的schema，即便是引用时该schema还没有被定义。

在下面的例子中，Author和Book对象是一对多的关系：

class AuthorSchema(Schema):
    # 必须使用only或exclude参数避免无限递归
    books = fields.Nested('BookSchema', many=True, exclude=('author', ))
    class Meta:
        fields = ('id', 'name', 'books')

class BookSchema(Schema):
    author = fields.Nested(AuthorSchema, only=('id', 'name'))
    class Meta:
        fields = ('id', 'title', 'author')

from marshmallow import pprint
from mymodels import Author, Book

author = Author(name='William Faulkner')
book = Book(title='As I Lay Dying', author=author)
book_result, errors = BookSchema().dump(book)
pprint(book_result, indent=2)
# {
#   "id": 124,
#   "title": "As I Lay Dying",
#   "author": {
#     "id": 8,
#     "name": "William Faulkner"
#   }
# }

author_result, errors = AuthorSchema().dump(author)
pprint(author_result, indent=2)
# {
#   "id": 8,
#   "name": "William Faulkner",
#   "books": [
#     {
#       "id": 124,
#       "title": "As I Lay Dying"
#     }
#   ]
# }

也可以使用导入模块的方式传递嵌套schema，如books = fields.Nested('path.to.BookSchema', many=True, exclude=('author', ))

schema自嵌套

给Nested传递字符串参数self表示和对象本身的关系：

class UserSchema(Schema):
    name = fields.String()
    email = fields.Email()
    friends = fields.Nested('self', many=True)
    # Use the 'exclude' argument to avoid infinite recursion
    employer = fields.Nested('self', exclude=('employer', ), default=None)

user = User("Steve", '[e-mail]')
user.friends.append(User("Mike", '[e-mail]'))
user.friends.append(User('Joe', '[e-mail]'))
user.employer = User('Dirk', '[e-mail]')
result = UserSchema().dump(user)
pprint(result.data, indent=2)
# {
#     "name": "Steve",
#     "email": "[e-mail]",
#     "friends": [
#         {
#             "name": "Mike",
#             "email": "[e-mail]",
#             "friends": [],
#             "employer": null
#         },
#         {
#             "name": "Joe",
#             "email": "[e-mail]",
#             "friends": [],
#             "employer": null
#         }
#     ],
#     "employer": {
#         "name": "Dirk",
#         "email": "[e-mail]",
#         "friends": []
#     }
# }