Fix query lookup when using Money object

[VictorTan93]

📝 Description

Fix query lookup when using Money object

This PR fixes an issue with Django ORM query lookups when using Money objects directly in filters.

Previously, querying with a Money object (e.g. Model.objects.filter(price=Money(100, 'USD'))) would only compare the amount without checking the currency, leading to incorrect results when multiple currencies are present in the database.

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📝 Changes

Core Fix:

Implemented custom lookup classes (MoneyExactLookup, MoneyLtLookup, MoneyLteLookup, MoneyGtLookup, MoneyGteLookup):

• Properly handle Money objects by checking both amount and currency in the SQL query
• Reference: https://docs.djangoproject.com/en/4.0/howto/custom-lookups/

Moving away from MoneyManager approach, because it is easily overwritten when a custom manager is used in a Django Model (i.e. objects = CustomManager()).

Code Cleanup:

• Removed MoneyManager class
• Extracted MoneyFieldProxy into its own module (proxy.py) for better organization
• Extracted utility functions (currency_field_name, currency_field_db_column) into utils.py

Testing:

Added comprehensive test suite in test_django_lookups.py covering:

• Exact lookups with Money objects
• Greater than/less than comparisons
• Greater than or equal to / less than or equal to comparisons
• Edge cases and currency validation

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Behavior

When filtering with a Money object:

Product.objects.filter(price=Money(100, 'USD'))

The generated SQL now checks both the amount column AND the currency column:

WHERE (price = 100 AND price_currency = 'USD')

This ensures that only records with matching currency are returned, preventing incorrect comparisons between amounts in different currencies.

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✅ Pre-review checklist for the PR author

• I have tested my code locally on my machine
• I have written tests (if appropriate)
• I have added appropriate label to PR

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[VictorTan93]

Moved to test_django_lookups.py

[ewendlick]

NAB: Very minor, but what do you think about calling this something a little different, like contains_currency_field, has_currency_field, with_currency_field, or using_currency_field?

"add_currency_field" feels a little ambiguous to me, whether it is for adding a currency field to an existing instance, for positive amounts, or if it is a method

[VictorTan93]

Ahhh this is not introduced by me, this line was added for mypy typing.
Changing this will affect everyone using it