Import Flask
and SQLAlchemy
modules first:
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
Declare the app
and db
objects:
app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///inquestion.db'
db = SQLAlchemy(app)
There are three tables: Artist
, Album
and Genre
. The Artist
object can be linked to multiple Albums
. And the Album
object can be linked to multiple Artists
. The albums_to_artists_table
is to keep the relationship between the Artists
and Albums
tight:
albums_to_artists_table = db.Table('albums_to_artists_table',
db.Column('album_id', db.Integer, db.ForeignKey('album.id')),
db.Column('artist_id', db.Integer, db.ForeignKey('artist.id')))
class Genre(db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(80), unique=True)
class Album(db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(80), unique=True)
genre_id = db.Column(db.Integer, db.ForeignKey('genre.id'))
artists = db.relationship('Artist', backref='albums', lazy='dynamic', secondary=albums_to_artists_table)
class Artist(db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(80), unique=True)
_albums = db.relationship('Album', secondary=albums_to_artists_table, backref=db.backref('albums_to_artists_table_backref', lazy='dynamic'))
So we have the Artist
linked to the Album
which is linked to Genre
and it looks like this: Artist
> Album
> Genre
.
Having this setup in place we go ahead and create the Genre
object first:
db.drop_all()
db.create_all()
genre = Genre(name='Heavy Metal')
db.session.add(genre)
db.session.commit()
Then two albums:
album1 = Album(name='Ride the Lightning', genre_id = genre.id)
album2 = Album(name='Master of Puppets ', genre_id = genre.id)
db.session.add(album1)
db.session.add(album2)
db.session.commit()
And the artist:
artist = Artist(name='Metallica', _albums=[album1, album2])
db.session.add(artist)
db.session.commit()
After the database created we can query what Albums
are linked to Genre
:
print Album.query.filter_by(genre_id=1).all()
and what Artists
are linked to Album
:
print Artist.query.filter(Artist._albums.any(id=album1.id)).all()
Now I would like to query all the Artists
that are linked to a Genre
passing the genre.id. How to achieve it?
See Question&Answers more detail:
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