I am trying to do a simple join query like this,
SELECT food._id, food.food_name, food_categories.food_categories FROM food JOIN food_categories ON food.food_category_id = food_categories._id
but keep receiving an error. Here is how my classes are setup.
class Food_Categories(db.Model):
__tablename__ = 'food_categories'
_id = db.Column(db.Integer, primary_key=True)
food_categories = db.Column(db.String(30))
class Food(db.Model):
__tablename__ = 'food'
_id = db.Column(db.Integer, primary_key=True)
food_name = db.Column(db.String(40))
food_category_id = db.Column(db.Integer, ForeignKey(Food_Categories._id))
food_category = relationship("Food_Categories")
My query function looks like this.
@app.route('/foodlist')
def foodlist():
if request.method == 'GET':
results = Food.query.join(Food_Categories.food_categories).all()
json_results = []
for result in results:
d = {'_id': result._id,
'food': result.food_name,
'food_category': result.food_categories}
json_results.append(d)
return jsonify(user=json_results)
I am using Flask. When I call the route I get this error.
AttributeError: 'ColumnProperty' object has no attribute 'mapper'
I essentially want this:
| id | food_name | food_category |
and have the food_category_id column replaced with the actual name of the food category located in other table.
Are my tables/relationships set up correctly? Is my query setup correctly?
Your tables and relationships are setup correctly. Your query needs a change.
The reason for an error is the fact that you try to perform a join on the column (Food_Categories.food_categories
) instead of a Table (or mapped model object). Technically, you should replace your query with the one below to fix the error:
results = Food.query.join(Food_Categories).all()
This will fix the error, but will not generate the SQL
statement you desire, because it will return instances of Food
only as a result even though there is a join.
In order to build a query which will generate exactly the SQL
statement you have in mind:
results = (db.session.query(Food._id, Food.food_name,
Food_Categories.food_categories,)
.join(Food_Categories)).all()
for x in results:
# print(x)
print(x._id, x.food_name, x.food_categories)
Please note that in this case the results are not instances of Food
, but rather tuples
with 3 column values.
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