Flask-sqlalchemy: How serialize objects with custom constructor from existing database? - python-3.x

Question

I'm trying to learn how to create python-based back-ends from some existing data that i have collected. I've come to realize that i definitely want to use sqlalchemy and that flask seems like a good library to go with it. My problem is that even after many hours of reading the sqlalchemy docs and browsing various answers on stackexchange i still don't understand how i can reshape data from an existing table into an object with a completely different structure.
The transformation i want to do is very concrete. I want to go from this structure in my MariaDB table:
Columns: company_name, date, indicators(1...23)
To this json output generated from a serialized class object:
{
"company_name[1]":
{
"indicator_name[1]":
{
"date[1]": "indicator_name[1].value[1]",
"date[2]": "indicator_name[1].value[2]",
"date[3]": "indicator_name[1].value[3]",
"date[4]": "indicator_name[1].value[4]",
"date[5]": "indicator_name[1].value[5]"
},
"indicator_name[2]":
{
"date[1]": "indicator_name[2].value[1]",
"date[2]": "indicator_name[2].value[2]",
"date[3]": "indicator_name[2].value[3]",
"date[4]": "indicator_name[2].value[4]",
"date[5]": "indicator_name[2].value[5]"
},
I found a great tutorial with which i can output the entire table record by record but the structure is not what i want, and i don't think creating the desired structure on the front-end makes sense in this case.
Here is the code that outputs the entire table to json record by record:
from flask import Flask, jsonify
from flask_sqlalchemy import SQLAlchemy
from sqlalchemy import PrimaryKeyConstraint
from sqlalchemy import orm
from sqlalchemy import select, func
from sqlalchemy import Column, Integer, String, ForeignKey
from flask_marshmallow import Marshmallow
import decimal
import flask.json
class MyJSONEncoder(flask.json.JSONEncoder): # Enables decimal queries for the API
def default(self, obj):
if isinstance(obj, decimal.Decimal):
# Convert decimal instances to strings.
return str(obj)
return super(MyJSONEncoder, self).default(obj)
app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'mysql+pymysql://USER:PASS#localhost:3306/kl_balance_sheets'
app.json_encoder = MyJSONEncoder
db = SQLAlchemy(app)
ma = Marshmallow(app)
# Bind declarative base to engine
db.Model.metadata.reflect(db.engine)
class CompanyData(db.Model):
__table__ = db.Model.metadata.tables['kl_balance_sheets']
class CompanyDataSchema(ma.ModelSchema):
class Meta:
model = CompanyData
#app.route('/')
def index():
company_data = CompanyData.query.all()
company_data_schema = CompanyDataSchema(many=True)
output = company_data_schema.dump(company_data).data
return jsonify({'company_data' : output})
if __name__ == '__main__':
app.run(debug=True)
My main question i guess is: How do i edit this code to produce the desired json?
What i think i should do is to create a custom constructor and then feed that into the index function but i can't figure out how to concretely do that. The two options i've come across are:
#orm.reconstructor
def init_on_load(self):
#do custom stuff
or:
class Foo(db.Model):
# ...
def __init__(**kwargs):
super(Foo, self).__init__(**kwargs)
# do custom stuff
To me this seems like a basic operation any flask-marshmallow user would be doing regularly. Could someone please explain how sql data is normally inserted into an object with a new structure and then serialized? In my case, do i need to change things mainly on the metadata, object or marshmallow level? I'm surprised i can't find some good examples of this.