marshmallow之schema嵌套

摘要：嵌套可以嵌套使用以表示对象间的关系如外键关系。在下面的例子中，和对象是一对多的关系必须使用或参数避免无限递归也可以使用导入模块的方式传递嵌套，如自嵌套给传递字符串参数表示和对象本身的关系

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="monty@python.org")
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"monty@python.org",
#             "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)

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"monty@python.org"}
# }

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"monty@python.org"}
#     }
# }

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": "steve@example.com",
#     "friends": ["Mike", "Joe"]
# }

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", ))

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", "steve@example.com")
user.friends.append(User("Mike", "mike@example.com"))
user.friends.append(User("Joe", "joe@example.com"))
user.employer = User("Dirk", "dirk@example.com")
result = UserSchema().dump(user)
pprint(result.data, indent=2)
# {
#     "name": "Steve",
#     "email": "steve@example.com",
#     "friends": [
#         {
#             "name": "Mike",
#             "email": "mike@example.com",
#             "friends": [],
#             "employer": null
#         },
#         {
#             "name": "Joe",
#             "email": "joe@example.com",
#             "friends": [],
#             "employer": null
#         }
#     ],
#     "employer": {
#         "name": "Dirk",
#         "email": "dirk@example.com",
#         "friends": []
#     }
# }