keyword-based get() for composite primary key

Migrated issue, originally created by Alex Rothberg

Right now the ident parameter to query.get is a tuple in the case of composite primary keys with the stipulation:

For a composite primary key, the order of identifiers corresponds in most cases to that of the mapped :class:.Table object’s primary key columns.

Having to know and hard code the order of the primary keys seems less than ideal. I suggest accepting a dict mapping names to the values for the primary keys. My implementation of this looks like:

from sqlalchemy.inspection import inspect

def get_by_pks(model, **kwargs):
    return model.query.get(
        tuple(
            kwargs[key.name]
            for key in inspect(model).primary_key
        )
    )

[zzzeek]

@sanjana96 that is the general idea, although it requires a few changes. if you can submit as a PR I can illustrate; the conversion from mapper.primary_key needs to be stated in terms of attribute names, not column keys, and I’d like to see if there’s a way to implement the test for length only once rather than stating it twice (easier to maintain and cover in tests).

[zzzeek]

Here’s an MCVE that works, try this

from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import PrimaryKeyConstraint
from sqlalchemy import String
from sqlalchemy.orm import declarative_base
from sqlalchemy.orm import Session

Base = declarative_base()


class GuaranteedValue(Base):
    __tablename__ = "GUARANTEEDVALUE"
    __table_args__ = (PrimaryKeyConstraint("SPEC", "NAME"),)
    SPEC = Column(String)
    NAME = Column(String)


e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

s = Session(e)
s.add(GuaranteedValue(SPEC="spec", NAME="name"))
s.commit()

gv = s.get(GuaranteedValue, {"SPEC": "spec", "NAME": "name"})