mypy plugin: infer relationship information for models specified as strings

Is your feature request related to a problem? Please describe.

Currently the mypy plugin can infer types/information for a relationship when the model parameter is an actual class value, but cannot do so when it is a string:

from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import as_declarative
from typing import List, Optional

@as_declarative()
class Base:
    pass

class Foo(Base):
    id = Column(Integer(), primary_key=True)
    bars = relationship("Bar", back_populates="foo", uselist=True)
    # error, must be: bars: List["Bar"] = relationship("Bar", back_populates="foo", uselist=True)

class Bar(Base):
    id = Column(Integer(), primary_key=True)
    foo_id: Optional[int] = Column(ForeignKey(Foo.id))
    foo = relationship(Foo, back_populates="bars", uselist=False)

# successful inference:
reveal_type(Bar().foo) # Union[test2.Foo, None]

Describe the solution you’d like

In https://github.com/sqlalchemy/sqlalchemy/discussions/6372, we discussed that string parameters could be handled via doing a name lookup in the current module, similar to the plugin in https://github.com/dropbox/sqlalchemy-stubs. This requires the other model name being imported (potentially in an if TYPE_CHECKING: block), but that’s okay: doing the explicit type annotation also requires the name being in scope for mypy.

(This approach could potentially be used for Column(ForeignKey("Abc.def"), ...) too, but it was mentioned in #6372 that doing the .def lookup might be hard.)

Describe alternatives you’ve considered

The current situation of annotating the type explicitly is a little suboptimal, as the plugin doesn’t validate, that, for instance, the collection types are correct. Another option would to have the righthand side be partially inferred, e.g. list[_] and then do a type join between LHS and RHS; if the LHS annotation is list[Bar], then the join will be successful, but if it’s very wrong (e.g. just Bar), then the join(Bar, list[_]) will not be possible an error can be reported.

Additional context N/A

Have a nice day! you too 😄

[CaselIT]

I guess we can close as will not fix since v2 will not use the plugin

[zzzeek]

it’s all in 2.0 now, docs are not up yet save for the quickstart at https://docs.sqlalchemy.org/en/20/orm/quickstart.html where you can see relationship() works in the most minimal way possible, inferring the class from the string annotation.