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?

gribvirus74 · Accepted Answer · 2021-03-08 23:14:14Z

3

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

edited Mar 8, 2021 at 23:14

answered Mar 8, 2021 at 22:27

gribvirus74

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1 Comment

gbrennon Over a year ago

thank u man! this was an amazing contribution!

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