orm – Super9

Serialize and Deserialze Sqlalchemy Nested Objects with JSON

2017 年 11 月 1 日
Python

At first we build a relationship :

from sqlalchemy import Base, Unicode

class County(Base):

__tablename__ = 'county'

name = Column(Unicode(5))

towns = relationship('Town')

class Town(Base):

__tablename__ = 'town'

name = Column(Unicode(5))

county = relationship('County', back_populates='towns')

and we want to dump a county instance to json like this:

1	{"name": "台北市", "towns": [{"name": "大安區"},{"name": "文山區"}]}

extending JSONEncoder:

class Encoder(JSONEncoder):

def default(self, obj):

d = {}

if isinstance(obj, County):

return {'name': obj.name, 'towns': obj.towns}

if isinstance(obj, Town):

return {'name': obj.name}

d.update(obj.__dict__)

return d

1	json.dumps(county_instance, cls=Encoder)

if you want to decode your json file, simply supply a function for object_hood attribute:

def hook(dict):

if dict.get('towns'):

county = County()

county.__dict__.update(dict)

return county

else:

town = Town()

town.__dict__.update(dict)

return town

1	json.load(file, object_hook=hook)

object_hook is an optional function that will be called with the result of any object literal decoded (a dict). The return value of object_hook will be used instead of the dict. This feature can be used to implement custom decoders (e.g. JSON-RPC class hinting).

when a json file loads, we can print the parameter “dict” which pass in to object_hood function, the whole process will loads from the most inner object (a dict) to the outer objects look like this:

{"name": "文山區"}

{"name": "大安區"}

{"name": "台北市", "towns": [

<Town object at 0x0000018FFE817B38>,

]}