Using neomodel with Flask

appmodels.py

import neomodel

class Item(neomodel.StructuredNode):
    __primarykey__ = "name"
    name = neomodel.StringProperty(unique_index=True)

main_naive.py

"""
A simple example showing how using neomodel with Flask in a naive way 
can lead to exceptions.

This example is based on code found in this stack-overflow post:
https://stackoverflow.com/questions/52877575/flask-and-neomodel-modeldefinitionmismatch

!!! WARNING !!!
* Prior to running this example, please make sure that the NEO4J_BOLT_URL environment 
variable, points to a working and properly setup instance of a neo4j dbms.

///FOLLOW THE CODE LINE-TO-LINE TO GET A COHERENT NARRATIVE FROM THE INLINE COMMENTS///

"""

import os
import json
import flask
import neomodel

class Item(neomodel.StructuredNode):
    __primarykey__ = 'name'
    name = neomodel.StringProperty(unique_index=True)


def create_an_item():
    neomodel.db.set_connection(os.environ['NEO4J_BOLT_URL'])
    Item(name='Something Something Dark Side').save()
    
    
def create_app():
    """
    This is the original way presented to create a class. Please note
    the inline comments throughout
    """
    # In this example a Flask application is created explicity.
    app = flask.Flask(__name__)    
    neomodel.config.DATABASE_URL = os.environ['NEO4J_BOLT_URL']
    
    # At this point, the neomodel.db object has been initialised as expected
    @app.route('/', methods=['GET'])
    def get_all_items():
        # At the time this function gets to be called, it operates in a different 
        # process and therefore the neomodel.db that is used implicitly in retrieving
        # the Item from the database is completely different than the neomodel.db that 
        # that was initialised in the process of execution that created the app object.
        # Anything running within the request serving functions is entirely independent stand-alone code.
        return flask.jsonify({'items': [item.name for item in Item.nodes]})

    return app


if __name__ == '__main__':
    # !!!PRIOR TO RUNNING THE APP, CREATE AT LEAST ONE Item IN THE DATABASE!!!
    create_an_item()
    # Comment the above line and uncomment the following two to launch the app
    # flask_app = create_app()
    # At this point, the neomodel.db object used in this process is configured and valid
    # flask_app.run()
    # At this point, the neomodel.db object stays configured and valid
    # IN THIS THREAD OF EXECUTION
    #
    # For every subsequent request, a completely new process
    # is created. The neomodel.db object within THAT thread of execution 
    # would not have undergone the kind of preparation it has for this 
    # process. Consequently, its _NODE_CLASS_REGISTRY would be empty 
    # and any attempt to instantiate objects from the app's model would 
    # fail.

main_naive_fix.py

"""
Continuing from the above example, here is the _naive_ way of fixing the problem.

What is different in this script is the way that the neomodel.db object is being managed.

!!! WARNING !!!
* Prior to running this example, please make sure that the NEO4J_BOLT_URL environment 
variable, points to a working and properly setup instance of a neo4j dbms.

///FOLLOW THE CODE LINE-TO-LINE TO GET A COHERENT NARRATIVE FROM THE INLINE COMMENTS///

"""

import os
import json
import flask
import neomodel
# !!! NOTICE REMOVAL OF THIS INSTANTIATION STEP !!!
# import appmodels


def create_an_item():
    neomodel.db.set_connection(os.environ['NEO4J_BOLT_URL'])
    appmodels.Item(name='Something Something Dark Side').save()
    
    
def create_app():
    """
    This is the original way presented to create a class. Please note
    the inline comments throughout
    """
    # In this example a Flask application is created explicity.
    app = flask.Flask(__name__)    
    neomodel.config.DATABASE_URL = os.environ['NEO4J_BOLT_URL']
    
    # At this point, the neomodel.db object has been initialised as expected
    @app.route('/', methods=['GET'])
    def get_all_items():
        # !!! NOTICE ADDITION OF THIS INITIALISATION STEP !!! 
        import appmodels
        return flask.jsonify({'items': [item.name for item in appmodels.Item.nodes]})
        
    return app


if __name__ == "__main__":
    # !!!PRIOR TO RUNNING THE APP, CREATE AT LEAST ONE Item IN THE DATABASE!!!
    create_an_item()
    # Comment the above line and uncomment the following two to launch the app
    # flask_app = create_app()
    # At this point, the neomodel.db object is configured and valid
    # flask_app.run()