Querying Flask-SQLAlchemy through two table joins - Join not working - mysql

Question

I'm trying to join two tables.
Here are my tables simplified
class Spots(db.Model):
__tablename__ = "spots"
geobaseid = db.Column((db.Integer), primary_key=True,autoincrement=True)
spot = db.Column(db.String(50))
url_slug = db.Column(db.String(50))
region = db.Column(db.String(50))
country = db.Column(db.String(50))
chop = db.Column(db.Integer)
small_wave = db.Column(db.Integer)
flat = db.Column(db.Integer)
big_wave = db.Column(db.Integer)
west_direction = db.Column(db.Integer)
southwest_direction = db.Column(db.Integer)
amount = db.Column(db.Integer)
url_slug = db.Column(db.String(50))
forcast = db.relationship('Forcast_short')
class Forcast_short(db.Model):
__tablename__ = "forcast_short"
id = db.Column((db.Integer), primary_key=True,autoincrement=True)
date = db.Column(db.Date)
geobaseid = db.Column((db.Integer), ForeignKey('spots.geobaseid'))
spot = db.Column(db.String(50))
region = db.Column(db.String(50))
country = db.Column(db.String(50))
latitude = db.Column(db.Numeric(10,8))
longitude = db.Column(db.Numeric(10,8))
deg = db.Column(db.Numeric(65,1))
mps = db.Column(db.Numeric(65,1))
Here's my query that is not working
forcast_query = Forcast_short.query.join(Spots, Spots.geobaseid==Forcast_short.geobaseid).filter(Forcast_short.region == region).all()
What am I doing wrong?
When I run the query I only get results from Forcast_short with and without the filter.
<tbody>
<tr>{%for row in forcast_query%}
<td> {{row.spot}} </td>
<td>{{row.mps}} </td>
<td>{{row.url_slug}} </td>
</tr>
{%endfor%}
</tbody>
This query works in Mysql workbench.
select * from
(
SELECT * FROM sc_db2.forcast_short
) a
join
(
select * from sc_db2.spots
) b
on a.geobaseid = b.geobaseid
;

Answer 1

The way you made your models is quite confusing. In keeping with the documentation of sqlalchemy on Relationship Patterns, which I advise you to read, here is how you should set up a one to many relationship:
class Parent(Base):
__tablename__ = 'parent'
id = Column(Integer, primary_key=True)
children = relationship("Child")
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id'))
This relationship places a foreign key on the child table referencing the parent. relationship() is then specified on the parent, as referencing a collection of items represented by the child.
ABOUT YOUR QUERY:
After harmonizing your models, your request should look like this:
forcast_query = Forcast_short.query.join(Spots, Spots.geobaseid==Forcast_short.geobaseid).filter(Forcast_short.region == region).filter(Forcast_short.date == date_1).all()

Answer 2

Here's a shorter example which illustrates a one-to-many relationship between spots and forecasts (which I think is what you're trying to do):
from app import db
class Spot(db.Model):
id = db.Column(db.Integer, primary_key=True)
geobaseid = db.Column(db.Integer)
forecasts = db.relationship('Forecast', backref='spot', lazy='dynamic')
class Forecast(db.Model):
id = db.Column(db.Integer, primary_key=True)
region = db.Column(db.String(50))
spot_id = db.Column(db.Integer, db.ForeignKey('spot.id'))
The db object is set up in the app package initialiser, following Miguel Grinberg's pattern:
from flask import Flask
from config import Config
from flask_sqlalchemy import SQLAlchemy
from flask_migrate import Migrate
app = Flask(__name__)
app.config.from_object(Config)
db = SQLAlchemy(app)
migrate = Migrate(app, db)
from app import models
Note that having set up the foreign key relationship, you don't need an explicit join to access the parent's fields from the child object - SQLAlchemy allows you to just use dot notation:
>>> from app import db
>>> from app.models import Spot, Forecast
>>> for spot in Spot.query.all():
... print(spot.id, spot.geobaseid)
...
1 1
2 2
>>> for forecast in Forecast.query.all():
... print(forecast.id, forecast.region, forecast.spot_id, forecast.spot.geobaseid)
...
1 Scotland 2 2
2 England 2 2