Clash Royale CLAN TAG#URR8PPP

Sqlalchemy AttributeError: 'property' object has no attribute 'translate'

Situation

Users make purchases, which are stored as transactions in 3 different tables (depending on the type). I need to calculate total amount of transactions/purchases of female and male users, so I need to look into all 3 tables.

For this I created a @property in the User table:

@property

@property
def count_credits_purchases(self):
trans = object_session(self).query(Transaction_1).filter(Transaction_1.type == "credits").with_parent(self).count()
trans_vk = object_session(self).query(Transaction_2).filter(Transaction_2.type == "credits").with_parent(self).count()
trans_stripe = object_session(self).query(Transaction_3).filter(Transaction_3.type == "credits").with_parent(self).count()
value = trans + trans_vk + trans_stripe
return int(value)


I am trying to calculate the total amount of purchases by using sqlalchemy func.sum():

func.sum()

total_purchases_males_credits = db_session.query(func.sum(Users.count_credits_purchases))
.filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == "1")
.scalar()


Problem

AttributeError: 'property' object has no attribute 'translate'


The translate method is a string method, what is happening here? I definitely return an integer in count_credits_purchases.

count_credits_purchases

I made a test and checking the value per user is always correct:

all_users = db_session.query(Users).limit(200)
for user in all_users:
print (user.count_credits_purchases) # gives correct result


I could make a variable and calculate it in the loop, but it is super unefficient and would need probably 1 hour if there are 50k users. I need to understand how to work with the @property attribute

@property

hybrid_property

2 Answers
2

As the docs Using Descriptors and Hybrids says you should be using hybrid_property to be able to use them in your queries.

hybrid_property

Take a look at example in docs:

class EmailAddress(Base):
__tablename__ = 'email_address'

id = Column(Integer, primary_key=True)

# name the attribute with an underscore,
# different from the column name
_email = Column("email", String)

# then create an ".email" attribute
# to get/set "._email"
@property
def email(self):
return self._email


While our EmailAddress object will shuttle the value through the email descriptor and into the _email mapped attribute, the class level EmailAddress.email attribute does not have the usual expression semantics usable with Query. To provide these, we instead use the hybrid extension

EmailAddress

EmailAddress.email

hybrid

sqlalchemy.orm.exc.UnmappedInstanceError: Class 'sqlalchemy.ext.declarative.api.DeclarativeMeta' is not mapped; was a class (stats_models.Users) supplied where an instance was required?

The best solution here is probably using @hybrid_property but I had problems to make it work.

@hybrid_property

I came up with a completely different solution, using a classical method. This was super fast and so far I dont see any downsides:

# Normal method to calculate | Best case would probably be @hybrid_method
def count_credits_purchases(self, start_date, end_date, gender):
trans = db_session.query(Transaction_1).filter(Transaction_1.type == "credits", Transaction_1.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
trans_vk = db_session.query(Transaction_2).filter(Transaction_2.type == "credits", Transaction_2.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
trans_stripe = db_session.query(Transaction_3).filter(Transaction_3.type == "credits", Transaction_3.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
value = trans + trans_vk + trans_stripe
return value


Call in python:

total_purchases_males_credits = Users().count_credits_purchases(start_date, end_date, "1")


I would still like to know how good ths approach is compared to hybrid_property?

EDIT:

Its also possible to use @hybrid_method:

@hybrid_method

@hybrid_method
def count_credits_purchases(self, start_date, end_date, gender):
trans = db_session.query(Transaction_1).filter(Transaction_1.type == "credits", Transaction_1.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
trans_vk = db_session.query(Transaction_2).filter(Transaction_2.type == "credits", Transaction_2.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
trans_stripe = db_session.query(Transaction_3).filter(Transaction_3.type == "credits", Transaction_3.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
value = trans + trans_vk + trans_stripe
return value


And use it:

total_purchases_males_credits = db_session.query(func.sum(Users.count_credits_purchases(start_date, end_date, "1"))).scalar()


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