问题
I just start a simple project called flask_wiki this days and I'm using some flask extensions as the follows:
- Flask-SQLAlchemy
- Flask-Restful
- MarshMallow
Well, I just discovered that the MarshMallow project provides a class called 'ModelSchema', which reads all fields from my SQLAlchemy Model and provide a fully automated (de)serialializer.
In my case, I created a 'GUID' Field which is RDBM agnostic and inserted it on my Sqlalchemy model as follows:
from flask.ext.sqlalchemy import SQLAlchemy
from flask_wiki.backend.custom_fields import GUIDField
class Page(db.Model):
"""
Implements the Page Model.
"""
guid = db.Column(GUIDField, primary_key=True, default=uuid.uuid4)
name = db.Column(db.String, nullable=False)
raw_content = db.Column(db.Text)
rendered_content = db.Column(db.Text)
def __repr__(self):
return self.__str__()
def __str__(self):
return self.name
The GUIDField is implemented is this way:
from sqlalchemy.types import TypeDecorator, CHAR
import uuid
class GUIDField(TypeDecorator):
# Platform independent GUID Implementation that uses little endianess.
impl = CHAR
def load_dialect_impl(self, dialect):
return dialect.type_descriptor(CHAR(32))
def process_bind_param(self, value, dialect):
if value is None:
return value
else:
if isinstance(value, uuid.UUID):
return value.bytes_le
def process_result_value(self, value, dialect):
if value is None:
return value
else:
return uuid.UUID(bytes_le=value)
The code for create test objects (through mixer) is working; all the guids are generated and verified correctly.
With this in mind, a just created the following MarshMallow Serializer Field:
from marshmallow import fields
import uuid
class GUIDSerializationField(fields.Field):
def _serialize(self, value, attr, obj):
if value is None:
return value
else:
if isinstance(value, uuid.UUID):
return str(value)
else:
return None
Finally, I created the SerializerClass:
from flask_wiki.backend.backend import marsh
from flask_wiki.backend.custom_serialization_fields import GUIDSerializationField
from flask_wiki.backend.models import Page
from marshmallow import fields
class PageSchema(marsh.ModelSchema):
class Meta:
model = Page
guid = GUIDSerializationField()
page_schema = PageSchema()
pages_schema = PageSchema(many=True)
I tried to use this last code with and without inserting the guid field, but in all cases the following error occurs:
Traceback (most recent call last):
File "/home/arthas/dev/flask-wiki/flask_wiki/wiki.py", line 3, in <module>
from flask_wiki.backend import backend
File "/home/arthas/dev/flask-wiki/flask_wiki/backend/__init__.py", line 1, in <module>
import flask_wiki.backend.routes
File "/home/arthas/dev/flask-wiki/flask_wiki/backend/routes.py", line 2, in <module>
from flask_wiki.backend.views import PageView
File "/home/arthas/dev/flask-wiki/flask_wiki/backend/views.py", line 3, in <module>
from flask_wiki.backend.serializers import pages_schema, page_schema
File "/home/arthas/dev/flask-wiki/flask_wiki/backend/serializers.py", line 7, in <module>
class PageSchema(marsh.ModelSchema):
File "/home/arthas/env/wiki/lib/python3.5/site-packages/marshmallow/schema.py", line 116, in __new__
dict_cls=dict_cls
File "/home/arthas/env/wiki/lib/python3.5/site-packages/marshmallow_sqlalchemy/schema.py", line 53, in get_declared_fields
declared_fields = mcs.get_fields(converter, opts)
File "/home/arthas/env/wiki/lib/python3.5/site-packages/marshmallow_sqlalchemy/schema.py", line 77, in get_fields
return converter.fields_for_model(opts.model, fields=opts.fields, exclude=opts.exclude)
File "/home/arthas/env/wiki/lib/python3.5/site-packages/marshmallow_sqlalchemy/convert.py", line 75, in fields_for_model
field = self.property2field(prop)
File "/home/arthas/env/wiki/lib/python3.5/site-packages/marshmallow_sqlalchemy/convert.py", line 93, in property2field
field_class = self._get_field_class_for_property(prop)
File "/home/arthas/env/wiki/lib/python3.5/site-packages/marshmallow_sqlalchemy/convert.py", line 151, in _get_field_class_for_property
field_cls = self._get_field_class_for_column(column)
File "/home/arthas/env/wiki/lib/python3.5/site-packages/marshmallow_sqlalchemy/convert.py", line 121, in _get_field_class_for_column
return self._get_field_class_for_data_type(column.type)
File "/home/arthas/env/wiki/lib/python3.5/site-packages/marshmallow_sqlalchemy/convert.py", line 143, in _get_field_class_for_data_type
'Could not find field column of type {0}.'.format(types[0]))
marshmallow_sqlalchemy.exceptions.ModelConversionError: Could not find field column of type <class 'flask_wiki.backend.custom_fields.GUIDField'>.
So, I finally ask: how to use marshmallow to serialize a custom sqlalchemy field?
回答1:
You need to create your own Converter. Try something like this:
import uuid
from flask_wiki.backend.models import Page
from flask_wiki.backend.backend import marsh
from marshmallow_sqlalchemy.convert import ModelConverter
from flask_wiki.backend.custom_fields import GUIDField
from marshmallow import fields
#Here you are overwriting the list of types of the SQLAlchemy, adding your custom type
class GUIDConverter(ModelConverter):
SQLA_TYPE_MAPPING = dict(
list(ModelConverter.SQLA_TYPE_MAPPING.items()) +
[(GUIDField, fields.Str)]
)
class GUIDSerializationField(fields.Field):
def _serialize(self, value, attr, obj):
if value is None:
return value
else:
if isinstance(value, uuid.UUID):
return str(value)
else:
return None
class PageSchema(marsh.ModelSchema):
class Meta:
model = Page
model_converter = GUIDConverter #Tell to Marshmallow to use your custom converter for this model
guid = GUIDSerializationField(attribute="guid")
You can also see this link for help. I hope it helped and sorry for my bad english
回答2:
According to what I've read in the docs, you can specify a model_converter attribute in your ModelSchema Meta class. The ModelConverter class has a SQLA_TYPE_MAPPING attribute you can override in a subclass to add your custom GUID field to the types detected by the automatic schema generator.
That said, I've never used it so I don't know if this will work or not.
来源：https://stackoverflow.com/questions/33160762/how-to-use-marshmallow-to-serialize-a-custom-sqlalchemy-field