Flask + SQLAlchemy result to JSON

Question

I'm new to Flask and SQLAlchemy and I have spent so much time trying to get my database query to present as JSON using a Flask url (not flask_restful).

I thought I had it working by using n0nSmoker/SQLAlchemy-serializer. It worked using a simple query using this syntax selecting my Trade SQLAlchemy class:

trades = Trade.query.all()

I was able to return trades.to_dict() with no problem.

However because I need to do a group_by, I need to use the more flexible syntax of db.session.query(Trade, Trade.exchange, Trade.trader....... etc this method stopped working.

I have looked at other options like the inspection system but it seems a bit beyond my skill level. Plus it seems like some of the methods like _asdict() aren't available at the row level.

I'm a bit lost and not sure how to proceed. Any help would be hugely appreciated.

Edit: More code requested: top of app.py

from flask import Flask
from flask_restful import Resource, Api
from models import *
from flask_sqlalchemy import SQLAlchemy
from sqlathanor import FlaskBaseModel, initialize_flask_sqlathanor

app = Flask(__name__)
app.config.from_pyfile('config.py')

db = SQLAlchemy(model_class = FlaskBaseModel)
db = initialize_flask_sqlathanor(db)

class Trades(Resource):
    def get(self):
        return AllTrades()

api.add_resource(Trades,'/')


if __name__ == '__main__':
    app.run(host="0.0.0.0")

from models.py

from app import db

class Trade(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    exchange = db.Column(db.String(255))
    trader = db.Column(db.String(255))
    symbol = db.Column(db.String(255))
    coin = db.Column(db.String(255))
    order_id = db.Column(db.String(255))
    price = db.Column(db.String(255))
    qty = db.Column(db.String(255))
    fee = db.Column(db.String(255))
    fee_currency = db.Column(db.String(255))
    order_time = db.Column(db.DateTime(timezone=False))
    side = db.Column(db.String(255))
    maker_taker = db.Column(db.String(255))


def AllTrades():
    #t = Trade
    # trades = db.session.query(
    #   t,
    #   t.exchange,
    #   t.trader,
    #   t.symbol,
    #   t.coin,
    #   db.func.sum(t.qty),
    #   db.func.sum(t.fee).label("fee"),
    #   t.fee_currency,
    #   t.order_id,
    #   t.order_time,
    #   t.maker_taker
    # ).group_by(t.order_id).all()

    trades = Trade.query.all()
    trades = trades.to_json()

    return trades

Full stack trace

    Traceback (most recent call last):
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask/app.py", line 1997, in __call__
    return self.wsgi_app(environ, start_response)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask/app.py", line 1985, in wsgi_app
    response = self.handle_exception(e)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask_restful/__init__.py", line 273, in error_router
    return original_handler(e)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask/app.py", line 1540, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask/_compat.py", line 32, in reraise
    raise value.with_traceback(tb)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask/app.py", line 1982, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask/app.py", line 1614, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask_restful/__init__.py", line 273, in error_router
    return original_handler(e)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask/app.py", line 1517, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask/_compat.py", line 32, in reraise
    raise value.with_traceback(tb)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask/app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask/app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask_restful/__init__.py", line 480, in wrapper
    resp = resource(*args, **kwargs)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask/views.py", line 84, in view
    return self.dispatch_request(*args, **kwargs)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask_restful/__init__.py", line 595, in dispatch_request
    resp = meth(*args, **kwargs)
  File "/Users/foo/Sites/Reporting_dev/app.py", line 18, in get
    return AllTrades()
  File "/Users/foo/Sites/Reporting_dev/models.py", line 42, in AllTrades
    trades = Trade.query.all()
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask_sqlalchemy/__init__.py", line 514, in __get__
    return type.query_class(mapper, session=self.sa.session())
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/sqlalchemy/orm/scoping.py", line 74, in __call__
    return self.registry()
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/sqlalchemy/util/_collections.py", line 1001, in __call__
    return self.registry.setdefault(key, self.createfunc())
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 2939, in __call__
    return self.class_(**local_kw)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask_sqlalchemy/__init__.py", line 143, in __init__
    bind = options.pop('bind', None) or db.engine
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask_sqlalchemy/__init__.py", line 877, in engine
    return self.get_engine()
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask_sqlalchemy/__init__.py", line 887, in get_engine
    state = get_state(app)
  File "/Users/foo/anaconda3/lib/python3.6/site-packages/flask_sqlalchemy/__init__.py", line 570, in get_state
    'The sqlalchemy extension was not registered to the current ' \
AssertionError: The sqlalchemy extension was not registered to the current application.  Please make sure to call init_app() first.

Answer 1

I’d suggest you take a look at a new library I released a week or two ago called SQLAthanor.

It adds simple serialization support to SQLAlchemy models, and if you are also using Flask-SQLAlchemy, it works seamlessly with it as well.

Basically, the way it works is this:

1. You define your Trade model the way you’re (presumably) already doing. The only difference is you import declarative_base or your base model from SQLAthanor instead of from SQLAlchemy (or Flask-SQLAlchemy).

2. You configure your serialization rules when defining your model. This basically lets you say “when I serialize a Trade to JSON, include attributes X, Y, and Z but not P, D, and Q”.

3. Execute your query however you would normally. You’ll get back a list of Trade instances, presumably. You can now serialize each instance to JSON by calling <instance>.to_json().

It’s worth mentioning that SQLAthanor:

• supports serialization and de-serialization to/from JSON, CSV, YAML, and Python dict
• supports serialization/de-serialization of Column attributes, relationships, hybrid properties, association proxies and regular Python @properties
• supports custom pre/post processing functions for serialization/de-serialization at the attribute level (for validation or type coercion)
• supports enabling/disabling serialization and de-serialization at the attribute level for particular formats (for example, accept passwords inbound, but never include them outbound)

It might be worth taking a look. Here’s a link to the (I hope comprehensive) documentation: http://sqlathanor.readthedocs.io/en/latest/

Hope this helps, and if you have questions or need help getting it working, just let me know (or post questions to SO with the tag sqlathanor)