[Sqlalchemy-tickets] Issue #4302: startswith is a comparison (zzzeek/sqlalchemy)

[Charles Bouchard-L. <iss...@bi...>]

New issue 4302: startswith is a comparison
https://bitbucket.org/zzzeek/sqlalchemy/issues/4302/startswith-is-a-comparison

Charles Bouchard-Legare:

Since startswith is a kind of like and like is a comparison, startswith should be too.

This would enable startswith-based custom join conditions without the use of a custom op.

Without it, I get:

```
sqlalchemy.exc.ArgumentError: Relationship Order.project could not determine any unambiguous local/remote column pairs based on join condition and remote_side arguments.  Consider using the remote() annotation to accurately mark those elements of the join condition that are on the remote side of the relationship.
```

To reproduce, try the following models:

```
#!python
class Project(Base):
    __tablename__ = 'project'

    id = Column(Integer, primary_key=True)
    name = Column(String)


class Order(Base):
    __tablename__ = 'order'

    id = Column(Integer, primary_key=True)
    project_name = Column(String)

    project = relationship(
        'Project',
        primaryjoin='Order.project_name.startswith(foreign(Project.name))',
        viewonly=True
    )
```

Workaround : 

```
#!python
from sqlalchemy.sql import operators

operators._comparison.add(operators.startswith_op)
```

Test case could look like

```
#!python
def test():
    assert operators.is_comparison(operators.startswith_op)
```