Using a custom JSON encoder for SQLAlchemy's PostgreSQL JSONB implementation

Question

I am using SQLAlchemy\'s core library to access some PostgreSQL database. Consider I have the following table:

create table foo (j jsonb);

Answer 1

If you're using Flask, you already have an extended JSONEncoder defined in flask.json which handles UUID, but not Decimal. It can be mapped into the SqlAlchemy engine with the json_serializer param as in @univerio's answer:

from flask import json

engine = create_engine(
    app.config['SQLALCHEMY_DATABASE_URI'],
    convert_unicode=True,
    json_serializer=json.dumps,
)

You can further extend the Flask JSONEncoder to support decimal.Decimal with the following:

import decimal

from flask import json

class CustomJSONEncoder(json.JSONEncoder):
    """
    Override Flask's JSONEncoder with the single method `default`, which 
    is called when the encoder doesn't know how to encode a specific type.
    """
    def default(self, obj):
        if type(obj) is decimal.Decimal:
            return str(obj)
        else:
            # raises TypeError: obj not JSON serializable
            return json.JSONEncoder.default(self, obj)

def init_json(app):
    """
    Use custom JSON encoder with Flask
    """
    app.json_encoder = CustomJSONEncoder