Experimenting in Interactive Mode

We can experiment with our app using the interactive mode of the Python interpreter.

You've undoubtedly used Python interactive mode before, which you can get to by simply starting your terminal and entering python3 :

$ python3
Python 3.7.3 (default, Mar 27 2019, 16:54:48) 
[Clang 4.0.1 (tags/RELEASE_401/final)] :: Anaconda, Inc. on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 

Let's see how we can use this to experiment with our SQLAlchemy apps!

Code

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql://udacitystudios@localhost:5432/example'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
db = SQLAlchemy(app)

class Person(db.Model):
  __tablename__ = 'persons'
  id = db.Column(db.Integer, primary_key=True)
  name = db.Column(db.String(), nullable=False)

db.create_all()

@app.route('/')
def index():
  person = Person.query.first()
  return 'Hello ' + person.name

Follow along

Code

Try running this yourself in the terminal.

$ python3
>>> from flask_hello_app import Person, db
>>> Person.query.all()
>>> Person.query.first()
>>> query = Person.query.filter(Person.name == 'Amy')
>>> query.first()
>>> query.all()

Debugging

Code

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql://udacitystudios@localhost:5432/example'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
db = SQLAlchemy(app)

class Person(db.Model):
  __tablename__ = 'persons'
  id = db.Column(db.Integer, primary_key=True)
  name = db.Column(db.String(), nullable=False)

  def __repr__(self):
    return f'<Person ID: {self.id}, name: {self.name}>'

db.create_all()

@app.route('/')
def index():
  person = Person.query.first()
  return 'Hello ' + person.name

Now try this: create records in python's interactive mode using db.session.add()

There's another way of inserting records into our database, rather than entering a client like psql and using INSERT INTO SQL commands: we can call db.session from SQLAlchemy to create records using instances of our defined SQLAlchemy models.

In interactive mode, import db and your Person model.

$ cd YOUR_PROJECT_DIRECTORY
$ python3
>>> from flask_hello_app import db, Person

Then, create an instance of a Person , setting its attributes, and setting it equal to a variable person

>>> person = Person(name='Amy')

We're going to call db.session.add() , a method on the Session interface in SQLAlchemy, to add this object to a session,

>>> db.session.add(person)

This will queue up a INSERT INTO persons (name) VALUES ('Amy'); statement in a transaction that is managed by db.session .

We can then call db.session.commit()

>>> db.session.commit()

and that person record will now exist in our persons table, within our database! You can double-check this in psql by running a SELECT * from persons; command from psql.

In summary

We can insert new records into the database using SQLAlchemy by running

person = Person(name='Amy')
db.session.add(person)
db.session.commit()

which will build a transaction for inserting in a person instance in our model/table, and persist it to the database upon calling commit() .

Workspace Option

You can try out these steps using the interactive workspace below, instead of using your own machine.