SQLAlchemy - How can I map a database value(column) to my value object class?

Question

how are u doing?

I've been developing some software using the classical mapping from SQLAlchemy and I want to know how can I map a database value to my own value object class.

For example, I have a Wallet class that has a Money attribute that is a value object.

domain

class Money(Decimal):
 # logic here
class Wallet:
 # other attributes
 balance: Money

mapper

wallet_table = Table(
 'wallet',
 metadata,
 Column('id', UUIDType, primary_key=True),
 Column('balance', Numeric, nullable=False)
)
wallet_mapper = mapper(Wallet, wallet_table)

How do I tell SQLAlchemy that when querying for this data on the database it should return balance as a Money?

Answer 1

You can do that with TypeDecorator:

from sqlalchemy.types import TypeDecorator, NUMERIC
class Money:
 value: float # Don't know the type you want but let's say it's a float
 # some logic
class MoneyDecorator(TypeDecorator):
 impl = NUMERIC
 def process_bind_param(self, money: Money, dialect) -> float:
 if money is not None:
 return money.value
 def process_result_value(self, value: float, dialect) -> Money:
 if value is not None:
 return Money(value)
wallet_table = Table(
 'wallet',
 metadata,
 Column('id', UUIDType, primary_key=True),
 Column('balance', MoneyDecorator, nullable=False)
)
wallet_mapper = mapper(Wallet, wallet_table)

Then you can do inserts and selects this way:

stmt = wallet_table.insert({'balance': Money(123.4)})
session.execute(stmt)
stmt = wallet_table.select()
result = sesion.execute(stmt)
print(result.first()['balance'].value) # Prints: 123.4

• gbrennon: thank u man! this was an amazing contribution!