Database and SQLAlchemy

In this blog we will explore using programs with data, focused on Databases. We will use SQLite Database to learn more about using Programs with Data. Use Debugging through these examples to examine Objects created in Code.

  • College Board talks about ideas like

    • Program Usage. "iterative and interactive way when processing information"
    • Managing Data. "classifying data are part of the process in using programs", "data files in a Table"
    • Insight "insight and knowledge can be obtained from ... digitally represented information"
    • Filter systems. 'tools for finding information and recognizing patterns"
    • Application. "the preserve has two databases", "an employee wants to count the number of book"

  • PBL, Databases, Iterative/OOP

    • Iterative. Refers to a sequence of instructions or code being repeated until a specific end result is achieved
    • OOP. A computer programming model that organizes software design around data, or objects, rather than functions and logic
    • SQL. Structured Query Language, abbreviated as SQL, is a language used in programming, managing, and structuring data

Imports and Flask Objects

Defines and key object creations

  • Comment on where you have observed these working? Provide a defintion of purpose.
    1. Flask app object
    2. SQLAlchemy db object
"""
These imports define the key objects
"""

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

"""
These object and definitions are used throughout the Jupyter Notebook.
"""

# Setup of key Flask object (app)
app = Flask(__name__)
# Setup SQLAlchemy object and properties for the database (db)
database = 'sqlite:///sqlite.db'  # path and filename of database
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
app.config['SQLALCHEMY_DATABASE_URI'] = database
app.config['SECRET_KEY'] = 'SECRET_KEY'
db = SQLAlchemy()


# This belongs in place where it runs once per project
db.init_app(app)

Notes and Observations #1

  • Imports Flask and SQLAlchemy libraries.

  • Creates a Flask application instance.

  • Sets up a database URI for an SQLite database

  • Disables SQLAlchemy modification tracking

  • Sets a secret key

  • Initializes a SQLAlchemy instance

  • Associates it with the Flask application.

  • Flask framework is used for creating web applications in Python.

  • Flask_SQLAlchemy extension is used to integrate SQLAlchemy with Flask.

  • SQLite database is used as the backend database.

  • SQLALCHEMY_TRACK_MODIFICATIONS configuration option is set to False, SQLAlchemy will not track modifications by default.

  • SECRET_KEY is set as a configuration option, which is used for secure session management.

  • db object is initialized and associated with the Flask application using init_app method.

Model Definition

Define columns, initialization, and CRUD methods for users table in sqlite.db

  • Comment on these items in the class, purpose and defintion.
    • class User
    • db.Model inheritance
    • init method
    • @property, @<column>.setter
    • create, read, update, delete methods
""" database dependencies to support sqlite examples """
import datetime
from datetime import datetime
import json

from sqlalchemy.exc import IntegrityError
from werkzeug.security import generate_password_hash, check_password_hash


''' Tutorial: https://www.sqlalchemy.org/library.html#tutorials, try to get into a Python shell and follow along '''

# Define the User class to manage actions in the 'users' table
# -- Object Relational Mapping (ORM) is the key concept of SQLAlchemy
# -- a.) db.Model is like an inner layer of the onion in ORM
# -- b.) User represents data we want to store, something that is built on db.Model
# -- c.) SQLAlchemy ORM is layer on top of SQLAlchemy Core, then SQLAlchemy engine, SQL
class User(db.Model):
    __tablename__ = 'users'  # table name is plural, class name is singular

    # Define the User schema with "vars" from object
    id = db.Column(db.Integer, primary_key=True)
    _name = db.Column(db.String(255), unique=False, nullable=False)
    _uid = db.Column(db.String(255), unique=True, nullable=False)
    _password = db.Column(db.String(255), unique=False, nullable=False)
    _dob = db.Column(db.Date)

    # constructor of a User object, initializes the instance variables within object (self)
    def __init__(self, name, uid, password="123qwerty", dob=datetime.today()):
        self._name = name    # variables with self prefix become part of the object, 
        self._uid = uid
        self.set_password(password)
        if isinstance(dob, str):  # not a date type     
            dob = date=datetime.today()
        self._dob = dob

    # a name getter method, extracts name from object
    @property
    def name(self):
        return self._name
    
    # a setter function, allows name to be updated after initial object creation
    @name.setter
    def name(self, name):
        self._name = name
    
    # a getter method, extracts uid from object
    @property
    def uid(self):
        return self._uid
    
    # a setter function, allows uid to be updated after initial object creation
    @uid.setter
    def uid(self, uid):
        self._uid = uid
        
    # check if uid parameter matches user id in object, return boolean
    def is_uid(self, uid):
        return self._uid == uid
    
    @property
    def password(self):
        return self._password[0:10] + "..." # because of security only show 1st characters

    # update password, this is conventional method used for setter
    def set_password(self, password):
        """Create a hashed password."""
        self._password = generate_password_hash(password, method='sha256')

    # check password parameter against stored/encrypted password
    def is_password(self, password):
        """Check against hashed password."""
        result = check_password_hash(self._password, password)
        return result
    
    # dob property is returned as string, a string represents date outside object
    @property
    def dob(self):
        dob_string = self._dob.strftime('%m-%d-%Y')
        return dob_string
    
    # dob setter, verifies date type before it is set or default to today
    @dob.setter
    def dob(self, dob):
        if isinstance(dob, str):  # not a date type     
            dob = date=datetime.today()
        self._dob = dob
    
    # age is calculated field, age is returned according to date of birth
    @property
    def age(self):
        today = datetime.today()
        return today.year - self._dob.year - ((today.month, today.day) < (self._dob.month, self._dob.day))
    
    # output content using str(object) is in human readable form
    # output content using json dumps, this is ready for API response
    def __str__(self):
        return json.dumps(self.read())

    # CRUD create/add a new record to the table
    # returns self or None on error
    def create(self):
        try:
            # creates a person object from User(db.Model) class, passes initializers
            db.session.add(self)  # add prepares to persist person object to Users table
            db.session.commit()  # SqlAlchemy "unit of work pattern" requires a manual commit
            return self
        except IntegrityError:
            db.session.remove()
            return None

    # CRUD read converts self to dictionary
    # returns dictionary
    def read(self):
        return {
            "id": self.id,
            "name": self.name,
            "uid": self.uid,
            "dob": self.dob,
            "age": self.age,
        }

    # CRUD update: updates user name, password, phone
    # returns self
    def update(self, name="", uid="", password=""):
        """only updates values with length"""
        if len(name) > 0:
            self.name = name
        if len(uid) > 0:
            self.uid = uid
        if len(password) > 0:
            self.set_password(password)
        db.session.commit()
        return self

    # CRUD delete: remove self
    # None
    def delete(self):
        db.session.delete(self)
        db.session.commit()
        return None

Notes and Observations #2

  • Defines a SQLAlchemy model class named User that represents a user in a web application.

    • User class has several properties and methods that define its behavior:

      • __tablename__ attribute: set to users,the name of the table in the database that corresponds to this model.

      • Properties that represent attributes of the user such as name, uid, password, dob, and age.

        • These properties have getter and setter methods.

      • __init__ method: defined to set default values for some of the attributes of the User class, such as password and dob. Initializes the class with values provided by the caller.

      • set_password method: sets a hash of the user's password using the werkzeug.security library.

      • is_password method: checks if a given password matches the user's stored password.

      • CRUD:

        • Adds a new user to the database.

        • Returns a dictionary representation of the user object.

        • Updates the properties of the user object and commits the changes to the database.

        • Deletes the user object from the database.

  • db object: a SQLAlchemy instance.

  • is_uid method not used in the code.

  • __str__ method: returns a JSON string representation of the user object.

  • IntegrityError exception is caught in the create method, indicating that a user with the same _uid attribute may already exist in the database.

Initial Data

Uses SQLALchemy db.create_all() to initialize rows into sqlite.db

  • Comment on how these work?
    1. Create All Tables from db Object
    2. User Object Constructors
    3. Try / Except 
"""Database Creation and Testing """


# Builds working data for testing
def initUsers():
    with app.app_context():
        """Create database and tables"""
        db.create_all()
        """Tester data for table"""
        u1 = User(name='Thomas Edison', uid='toby', password='123toby', dob=datetime(1847, 2, 11))
        u2 = User(name='Nikola Tesla', uid='niko', password='123niko')
        u3 = User(name='Alexander Graham Bell', uid='lex', password='123lex')
        u4 = User(name='Eli Whitney', uid='whit', password='123whit')
        u5 = User(name='Indiana Jones', uid='indi', dob=datetime(1920, 10, 21))
        u6 = User(name='Marion Ravenwood', uid='raven', dob=datetime(1921, 10, 21))


        users = [u1, u2, u3, u4, u5, u6]

        """Builds sample user/note(s) data"""
        for user in users:
            try:
                '''add user to table'''
                object = user.create()
                print(f"Created new uid {object.uid}")
            except:  # error raised if object nit created
                '''fails with bad or duplicate data'''
                print(f"Records exist uid {user.uid}, or error.")
                
initUsers()

Created new uid toby
Created new uid niko
Created new uid lex
Created new uid whit
Created new uid indi
Created new uid raven

Notes and Observations #3

  • Defines a function called initUsers().

    • Initializes user records in the database.

  • with app.app_context() sets the application context for the database.

  • db.create_all() creates the table if it doesn't exist yet.

  • Creates six user records with attributes such as name, uid, password, and date of birth.

    • If no password or dob is provided, the function sets default values.

  • user objects stored in a list called users.

  • Loop iterates over the users and attempts to create each one using the create() method of the User class.

    • If the create operation is successful, the function prints a message with the uid of the newly created user.

    • If the create operation fails due to an integrity error, the function prints a message indicating that the record already exists or an error occurred.

Check for given Credentials in users table in sqlite.db

Use of ORM Query object and custom methods to identify user to credentials uid and password

  • Comment on purpose of following
    1. User.query.filter_by
    2. user.password
def find_by_uid(uid):
    with app.app_context():
        user = User.query.filter_by(_uid=uid).first()
    return user # returns user object

# Check credentials by finding user and verify password
def check_credentials(uid, password):
    # query email and return user record
    user = find_by_uid(uid)
    if user == None:
        return False
    if (user.is_password(password)):
        return True
    return False
        
#check_credentials("indi", "123qwerty")

Notes and Observations #4

  • Defines two functions

    • find_by_uid

      • Takes a uid parameter and returns a User object with the matching _uid.

      • Uses Flask's application context to create a new context and then uses SQLAlchemy to query the User table for the matching _uid.

    • check_credentials

      • Takes a uid and password parameter and returns True if there is a user with matching _uid and password, and False otherwise.

      • Uses find_by_uid to get the User object with the matching _uid, and then checks if the given password matches the user's password using the is_password method defined in the User class.

Create a new User in table in Sqlite.db

Uses SQLALchemy and custom user.create() method to add row.

  • Comment on purpose of following
    1. user.find_by_uid() and try/except
    2. user = User(...)
    3. user.dob and try/except
    4. user.create() and try/except
def create():
    # optimize user time to see if uid exists
    uid = input("Enter your user id:")
    user = find_by_uid(uid)
    try:
        print("Found\n", user.read())
        return
    except:
        pass # keep going
    
    # request value that ensure creating valid object
    name = input("Enter your name:")
    password = input("Enter your password")
    
    # Initialize User object before date
    user = User(name=name, 
                uid=uid, 
                password=password
                )
    
    # create user.dob, fail with today as dob
    dob = input("Enter your date of birth 'YYYY-MM-DD'")
    try:
        user.dob = datetime.strptime(dob, '%Y-%m-%d').date()
    except ValueError:
        user.dob = datetime.today()
        print(f"Invalid date {dob} require YYYY-mm-dd, date defaulted to {user.dob}")
           
    # write object to database
    with app.app_context():
        try:
            object = user.create()
            print("Created\n", object.read())
        except:  # error raised if object not created
            print("Unknown error uid {uid}")
        
create()

Invalid date  require YYYY-mm-dd, date defaulted to 03-19-2023
Created
 {'id': 7, 'name': '', 'uid': '', 'dob': '03-19-2023', 'age': 0}

Notes and Observations #5

  • Defines a function create() that interacts with the user to create a new user in the database.

    • Prompts the user for their user ID and checks if the user already exists in the database using the find_by_uid() function.

      • If the user already exists, the function prints out the user's information and returns. Otherwise, the function prompts the user for their name and password, creates a new User object, and prompts the user for their date of birth.

        • If the date of birth is entered in an invalid format, the default date is set to the current date.

          • The new user is added to the database using the create() method of the User object.

            According to ChatGPT:an issue with the code is that it assumes that the find_by_uid() function always returns a valid User object. If the find_by_uid() function returns None, then an error will occur when trying to call the read() method on None. It would be better to check if the user variable is None before trying to print out the user's information.

Reading users table in sqlite.db

Uses SQLALchemy query.all method to read data

  • Comment on purpose of following
    1. User.query.all
    2. json_ready assignment, google List Comprehension
# SQLAlchemy extracts all users from database, turns each user into JSON
def read():
    with app.app_context():
        table = User.query.all()
    json_ready = [user.read() for user in table] # "List Comprehensions", for each user add user.read() to list
    return json_ready

read()

[{'id': 1,
  'name': 'Thomas Edison',
  'uid': 'toby',
  'dob': '02-11-1847',
  'age': 176},
 {'id': 2,
  'name': 'Nikola Tesla',
  'uid': 'niko',
  'dob': '03-19-2023',
  'age': 0},
 {'id': 3,
  'name': 'Alexander Graham Bell',
  'uid': 'lex',
  'dob': '03-19-2023',
  'age': 0},
 {'id': 4,
  'name': 'Eli Whitney',
  'uid': 'whit',
  'dob': '03-19-2023',
  'age': 0},
 {'id': 5,
  'name': 'Indiana Jones',
  'uid': 'indi',
  'dob': '10-21-1920',
  'age': 102},
 {'id': 6,
  'name': 'Marion Ravenwood',
  'uid': 'raven',
  'dob': '10-21-1921',
  'age': 101},
 {'id': 7, 'name': '', 'uid': '', 'dob': '03-19-2023', 'age': 0}]

Notes and Observations #6

  • Reads all the rows from the User table and returns a list of dictionary objects, where each dictionary contains user data in a format that can be easily converted to JSON:

    • Create a context for the Flask app.

    • Use the query.all() method of the User object to retrieve all rows from the User table.

    • Iterate over the rows and call the read() method of each row, which returns a dictionary object containing user data in a format that can be easily converted to JSON.

    • Return a list of all the dictionary objects.

Hacks

  • Add this Blog to you own Blogging site. In the Blog add notes and observations on each code cell.
  • Change blog to your own database.
  • Add additional CRUD
    • Add Update functionality to this blog.
    • Add Delete functionality to this blog.