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 or purpose.
    1. Flask app object
      • I have observed Flask app object in web applications built using Flask, which is a popular Python web framework. The Flask app object is the central point of a Flask application and is responsible for handling requests and responses. It is created by instantiating the Flask class and is configured with settings such as the name of the application, the location of the templates and static files, and the debug mode. The Flask app object is also responsible for registering the routes, views, and blueprints that define the behavior of the web application.
    2. SQLAlchemy db object
      • I have observed SQLAlchemy db object in web applications that use SQLAlchemy as an Object-Relational Mapping (ORM) tool. SQLAlchemy is a powerful library that enables developers to interact with databases in a more Pythonic way. The db object is typically used to represent the database connection and provides a simple and consistent interface to query the database. The db object is created by instantiating the SQLAlchemy class and is configured with the database URL, which specifies the location and credentials of the database. The db object provides a number of methods to interact with the database, such as querying the data, inserting new data, updating existing data, and deleting data. It also provides support for transactions, migrations, and other database-related tasks.
"""
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)

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
      • class User:The User class is a blueprint for creating user objects with attributes such as id, username, and password. It also contains methods for creating, reading, updating, and deleting user records in a database. - db.Model inheritance
      • db.Model inheritance: db.Model is a class provided by the Flask-SQLAlchemy extension that User class inherits from. This provides the ability to interact with the database using SQLAlchemy ORM (Object-Relational Mapping).
    • init method
      • init method: The init method is a special method in Python classes that is called when a new instance of the class is created. It is used to initialize the instance's attributes with the values passed in when the instance is created.
    • @property, @<column>.setter
      • @property, @.setter: These are Python decorators used to define getter and setter methods for class attributes. @property is used to define a getter method for a read-only attribute, while @.setter is used to define a setter method for an attribute mapped to a database column.</strong></li> </ul> </li>
      • create, read, update, delete methods
        • Create, read, update, delete methods: These are the standard CRUD (Create, Read, Update, Delete) operations used to manipulate data in a database. The create method creates a new record in the database, the read method retrieves a record from the database, the update method modifies an existing record in the database, and the delete method removes a record from the database.
        • </ul> </li> </ul> </div> </div> </div> 
        """ 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

        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
            • create_all() is a method in the SQLAlchemy db object that creates all the tables that have been defined in the database schema. It creates the tables by executing the SQL commands generated by the ORM, based on the definitions of the tables in the Python code. This method is usually called at the start of the application to ensure that all tables are created before the application starts.
          2. User Object Constructors
            • The User object constructor is a special method that is called when a new User object is created. It initializes the attributes of the User object with the values passed as arguments to the constructor. It can also perform additional initialization steps if needed.
          3. Try / Except
            • try and except are control flow statements in Python that allow us to handle exceptions that may be raised during the execution of the program. The try block contains the code that may raise an exception, and the except block contains the code that is executed if an exception is raised. If no exception is raised, the except block is skipped. This construct is used to handle errors and exceptions gracefully, without crashing the program.
        """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()

        Records exist uid toby, or error.
Records exist uid niko, or error.
Records exist uid lex, or error.
Records exist uid whit, or error.
Created new uid indi
Records exist uid raven, or error.

        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
            • User.query.filter_by is used in conjunction with an ORM (Object-Relational Mapping) like SQLAlchemy to retrieve data from a database. In this case, it is used to query the User table in the database and filter the results by a specific column, such as username.
          2. user.password
            • user.password refers to the password field of an instance of the User class. It is likely used to store a hashed version of the user's password in the database, which can then be compared to a hashed version of a user's inputted password during the login process to authenticate the user.
        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")

        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
            • user.find_by_uid() and try/except:user.find_by_uid() is likely a method used to retrieve a user object from some data storage (such as a database) using a unique identifier (UID). The purpose of using a try/except block around this method call is to handle exceptions that may occur if the user with the given UID is not found in the data storage. By wrapping the method call in a try block, we can catch the exception that is raised if the user is not found, and handle it in a more controlled way (such as returning a default value or raising a custom exception). 2. user = User(...)
            • creates a new User object and assigns it to the variable user. The ... would represent the parameters passed to the constructor of the User class, which could be attributes such as the user's name, email address, password, etc. The purpose of creating a User object is to represent a user in the application, allowing us to manipulate and access their data in a structured way.
          2. user.dob and try/except
            • refers to the "date of birth" attribute of the User object. The purpose of using a try/except block around this attribute access is to handle exceptions that may occur if the dob attribute is not set on the User object. By wrapping the attribute access in a try block, we can catch the exception that is raised if the dob attribute is not set, and handle it in a more controlled way (such as returning a default value or raising a custom exception).
          3. user.create() and try/except
            • a method used to create a new user in some data storage (such as a database). The purpose of using a try/except block around this method call is to handle exceptions that may occur during the creation process, such as database connection errors, data validation errors, or other unexpected errors. By wrapping the method call in a try block, we can catch the exception that is raised if an error occurs, and handle it in a more controlled way (such as rolling back the transaction or displaying an error message to the user).
        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.dbo}")
           
    # 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()

        Created
 {'id': 9, 'name': 'Samit Poojary', 'uid': 'samit', 'dob': '02-24-2006', 'age': 17}

        Reading users table in sqlite.db

        Uses SQLALchemy query.all method to read data

        • Comment on purpose of following
          1. User.query.all
            • User.query.all is a query method used to retrieve all objects from the User table in the database.
          2. json_ready assignment, google List Comprehension
            • json_ready is an assignment to prepare a list of dictionaries containing the required fields of each User object in a format suitable for JSON serialization. google List Comprehension is a method used to construct this list of dictionaries in a concise and efficient way.
        # 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] # each user adds 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': 6,
  'name': 'Marion Ravenwood',
  'uid': 'raven',
  'dob': '10-21-1921',
  'age': 101},
 {'id': 7,
  'name': 'jvo',
  'uid': 'crenshaw sheisty',
  'dob': '01-26-2006',
  'age': 17},
 {'id': 8,
  'name': 'Indiana Jones',
  'uid': 'indi',
  'dob': '10-21-1920',
  'age': 102},
 {'id': 9,
  'name': 'Samit Poojary',
  'uid': 'samit',
  'dob': '02-24-2006',
  'age': 17}]

        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.
        def delete_by_uid(): 
    uid = input("Enter uid of user to be deleted.")
    user = find_by_uid(uid)  # using previous function to locate user by inputted id
    with app.app_context():
        try:
            object = user.delete() 
            print(f"User with uid --{uid}-- has been deleted. ")
            db = read()
            print(db)
        except:  # error raised if object not found
           (f"No user with uid {uid} was found.")
        
delete_by_uid()

        User with uid --samit-- has been deleted. 
[{'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': 6, 'name': 'Marion Ravenwood', 'uid': 'raven', 'dob': '10-21-1921', 'age': 101}, {'id': 7, 'name': 'jvo', 'uid': 'crenshaw sheisty', 'dob': '01-26-2006', 'age': 17}, {'id': 8, 'name': 'Indiana Jones', 'uid': 'indi', 'dob': '10-21-1920', 'age': 102}]

        def update_user():
    uid = input("Enter uid of user to be updated")
    user = find_by_uid(uid)
    name = input("Enter new name:")
    password = input("Enter new password:")
    with app.app_context():
        try:
            object = user.update(name=name,uid=uid, password=password) 
            print(f"User with uid --{uid}-- has been updated: {object} ")
        except:  # error raised if object not found
           (f"No user with uid {uid} was found.")
    db = read()
    print("---Updated db:---")
    print(db)
update_user()

        User with uid --raven-- has been updated: {"id": 6, "name": "hummingbird", "uid": "raven", "dob": "10-21-1921", "age": 101} 
---Updated db:---
[{'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': 6, 'name': 'Marion Ravenwood', 'uid': 'raven', 'dob': '10-21-1921', 'age': 101}, {'id': 7, 'name': 'jvo', 'uid': 'crenshaw sheisty', 'dob': '01-26-2006', 'age': 17}, {'id': 8, 'name': 'Indiana Jones', 'uid': 'indi', 'dob': '10-21-1920', 'age': 102}]
        </div>