# sqlalchemy.exc.ArgumentError: Mapper Mapper|User|users could not assemble any primary key columns for mapped table 'users'

- **ID:** `python/sqlalchemy-missing-primary-key`
- **Domain:** python
- **Category:** config_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Model class lacks a primary key column definition.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.x | active | — | — |

## Workarounds

1. **Add a primary key column.** (95% success)
   ```
   class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
   ```
2. **Use __mapper_args__ to specify primary key from existing columns.** (90% success)
   ```
   class User(Base):
    __tablename__ = 'users'
    email = Column(String, primary_key=True)  # if email is unique
   ```

## Dead Ends

- **Adding a column without primary_key=True.** — SQLAlchemy requires explicit primary key constraint. (80% fail)
- **Using composite primary key with non-unique columns.** — Must ensure uniqueness; otherwise mapping fails. (60% fail)