MySQL JSON fields on the ground!

Having to add a series of custom fields to a quite relational application, I decided to try the new JSON fields.

As of now you can:

– create json fields
– manipulate them with json_extract, json_unquote
– create generated fields from json entries

You can not:

– index json fields directly, create a generated field and index it
– retain the original json datatype (eg. string, int), as json_extract always returns strings.

Let’s start with a simple flask app:

# requirements.txt
mysql-connector-python
Flask-SQLAlchemy==2.0
SQLAlchemy>=1.1.3

Let’s create a simple flask app connected to a db.

import flask
import flask_sqlalchemy
from sqlalchemy.dialects.mysql import JSON

# A simple flask app connected to a db
app = flask.Flask('app')
app.config['SQLALCHEMY_DATABASE_URI']='mysql+mysqlconnector://root:secret@localhost:3306/test'
db = flask_sqlalchemy.SQLAlchemy(app)

Add a class to the playground and create it on the db. We need sqlalchemy>=1.1 to support the JSON type!

# The model
class MyJson(db.Model):
    name = db.Column(db.String(16), primary_key=True)
    json = db.Column(JSON, nullable=True)

    def __init__(self, name, json=None):
        self.name = name
        self.json = json

# Create table
db.create_all()

Thanks to flask-sqlalchemy we can just db.session 😉

# Add an entry
entry = MyJson('jon', {'do': 'it', 'now': 1})
db.session.add(entry)
db.session.commit()

We can now verify using a raw select that the entry is now serialized on db

# Get entry in Standard SQL
entries = db.engine.execute(db.select(columns=['*'], from_obj=MyJson)).fetchall()
(name, json_as_string), = first_entry  # unpack result (it's just one!)
assert isinstance(json_as_string, basestring) 

A raw select to extract json fields now:

entries = db.engine.execute(db.select(columns=[name, 'json_extract(json, "$.now")'], from_obj=MyJson)).fetchall()

(name, json_now), = first_entry  # unpack result (it's just one!)
assert isinstance(json_now, basestring) 
assert json_now != entry.json['now']  # '1' != 1 

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