使用烧瓶棉花糖序列化几何图形

Question

I have a sqlalchemy2 (with geoalchemy extension) model containing geometry data type like so:

class Estate(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    geom = db.Column(Geometry(geometry_type='POLYGON')
    user_id = db.Column(db.Integer, db.ForeignKey('users.id'))
    user = db.relationship('Users', backref='estates')

and the marshmallow schema:

class EstateSchema(ma.ModelSchema):
    class Meta:
        model = Estates

but during attempt of serialization I received following error:

marshmallow_sqlalchemy.exceptions.ModelConversionError: Could not find field column of type <class 'geoalchemy2.types.Geometry'>.

The ModelConversion complaining about WKBElement , because is not defined as a common field.

Question is, how can I modify the code, to get the (de)serializer working with Geometry model field?

Answer 1

I know this is pretty old, but answering now as I just solved this for myself after landing on this question. The way I accomplished it is just by adding a method field to my schema like so:

from marshmallow_sqlalchemy import SQLAlchemySchema, fields
from geoalchemy2.shape import to_shape

from common.models import Node


class NodeSchema(SQLAlchemySchema):
    class Meta:
        model = Node

    geom = fields.fields.Method("geomToPoint")

    def geomToPoint(self, obj):
        return to_shape(obj.geom)

This returns the Geometry field as a Shapely shape field, something like POINT (39.2934324 -123.59348) which is what I was looking for.

Answer 2

marshmallow_sqlalchemy do not know how to serialize the field geom of type Geometry . This is because GeoAlchemy is not directly supported in marshmallow . In order to solve this you need to write a custom ModelConverter which tells marshmallow how to serialize a field of type Geometry .

from geoalchemy2.types import Geometry as GeometryType
from marshmallow_sqlchemy.convert import ModelConverter as BaseModelConverter


class ModelConverter(BaseModelConverter):
    SQLA_TYPE_MAPPING = {
        **BaseModelConverter.SQLA_TYPE_MAPPING,
        **{GeometryType: fields.Field()},
    }


class EstateSchema(ma.ModelSchema):
    class Meta:
        model = Estates
        model_converter = ModelConverter

The same has been mentioned in the github repository of marshmallow-sqlalchemy