Add soft deletion support to async-sqlalchemy-adapter

[Copilot]

Overview

This PR adds soft deletion support to the async-sqlalchemy-adapter, matching the functionality available in the synchronous sqlalchemy-adapter. This resolves the feature request in issue #XX.

What is Soft Deletion?

Instead of physically deleting policy records from the database, soft deletion marks them as deleted using a boolean flag. This preserves data history and enables auditing while maintaining the same Casbin API.

Implementation

Core Changes

• Added optional db_class_softdelete_attribute parameter to Adapter.__init__()
• Implemented _softdelete_query() helper method to filter out soft-deleted records automatically
• Updated all query operations (load_policy, load_filtered_policy, save_policy, etc.) to respect the soft delete flag
• Modified removal operations (remove_policy, remove_policies, remove_filtered_policy) to mark records as deleted instead of removing them

Usage

To use soft deletion, define a custom database model with a boolean is_deleted column:

from sqlalchemy import Column, Boolean
from casbin_async_sqlalchemy_adapter import Adapter, Base

class CasbinRuleSoftDelete(Base):
    __tablename__ = "casbin_rule"
    
    id = Column(Integer, primary_key=True)
    ptype = Column(String(255))
    v0 = Column(String(255))
    # ... other v columns ...
    
    # Add the soft delete column
    is_deleted = Column(Boolean, default=False, index=True, nullable=False)

# Create adapter with soft delete support
adapter = Adapter(
    'sqlite+aiosqlite:///test.db',
    db_class=CasbinRuleSoftDelete,
    db_class_softdelete_attribute=CasbinRuleSoftDelete.is_deleted
)

e = casbin.AsyncEnforcer('model.conf', adapter)

# Now when you remove policies, they will be soft-deleted
await e.remove_policy("alice", "data1", "read")
# The record remains in the database with is_deleted=True

Key Features

• ✅ Optional: Soft deletion is opt-in; fully backward compatible with existing code
• ✅ Type Safety: Validates that the soft delete column is of type Boolean
• ✅ Automatic Filtering: All queries automatically exclude soft-deleted records
• ✅ Data Preservation: Deleted records remain in database for audit purposes
• ✅ Transparent API: No changes to the Casbin API - works seamlessly
• ✅ Well Tested: Comprehensive test suite with 7 new tests covering all scenarios

Testing

All 26 tests pass:

• 15 existing tests (backward compatibility verified)
• 7 new soft deletion tests
• 4 external session tests

Documentation

Added complete documentation section in README.md with usage examples and feature description.

Breaking Changes

None. This is a fully backward-compatible addition. Existing code continues to work without any modifications.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Is there a plan to support soft deletion?</issue_title>
<issue_description>Is there a plan to support soft deletion? Similar to the soft deletion feature in sqlalchemy-adapter?</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #14

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