[sqlalchemy-spanner]: reflected primary keys have no "name" key, so Alembic >= 1.18 autogenerate fails with KeyError: 'name' #18841

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Summary of the issue

SpannerDialect.get_pk_constraint() and get_multi_pk_constraint() return primary keys as {"constrained_columns": [...]}, without the name key that SQLAlchemy's ReflectedPrimaryKeyConstraint requires (name: Optional[str], from ReflectedConstraint). Since Alembic 1.18.0, autogenerate reads that key, so alembic revision --autogenerate and alembic check fail with KeyError: 'name' on any Spanner database that has a table.

API client name and version

sqlalchemy-spanner 1.20.1 (also main at eb8a766)

Problem description

Alembic 1.18.0 (sqlalchemy/alembic#1771) changed autogenerate on SQLAlchemy 2.x to load the reflection data of all tables in a schema with the get_multi_*() inspector methods. It then wraps each reflected constraint name in conv(), which reads const["name"] from the primary key, foreign key, index and unique constraint dicts (_apply_reflectinfo_conv in alembic/autogenerate/compare/util.py, unchanged through 1.20.1). The primary key dicts from get_multi_pk_constraint() have no name, so this raises KeyError. Every Spanner table has a primary key, so autogenerate fails as soon as the database has a table.

Alembic 1.17 and earlier reflect one table at a time through Inspector.reflect_table(), which reads the name with .get("name"), so they don't hit this.

The compliance suite doesn't catch it, because the Spanner override of _check_list in tests/test_suite_20.py only compares keys that are in both the result and the expected dict.

Reproduction steps: code

Run against the emulator with alembic>=1.18 and sqlalchemy>=2.0:

import os

os.environ.setdefault("SPANNER_EMULATOR_HOST", "localhost:9010")

import sqlalchemy as sa
from alembic.autogenerate import compare_metadata
from alembic.ddl.impl import DefaultImpl
from alembic.migration import MigrationContext
from google.cloud import spanner


class SpannerImpl(DefaultImpl):
    __dialect__ = "spanner+spanner"


client = spanner.Client(project="emulator-test-project")
instance = client.instance(
    "test-instance", f"{client.project_name}/instanceConfigs/emulator-config"
)
if not instance.exists():
    instance.create().result()
database = instance.database(
    "pk-name-repro",
    ddl_statements=["CREATE TABLE singers (id INT64 NOT NULL) PRIMARY KEY (id)"],
)
if database.exists():
    database.drop()
database.create().result()

engine = sa.create_engine(
    "spanner+spanner:///projects/emulator-test-project/instances/test-instance"
    "/databases/pk-name-repro"
)
metadata = sa.MetaData()
sa.Table("singers", metadata, sa.Column("id", sa.BigInteger, primary_key=True))

with engine.connect() as connection:
    print(sa.inspect(connection).get_pk_constraint("singers"))
    print(compare_metadata(MigrationContext.configure(connection), metadata))

Reproduction steps: actual results

{'constrained_columns': ['id']}
Traceback (most recent call last):
  ...
  File ".../alembic/autogenerate/compare/tables.py", line 193, in _compare_tables
    _InspectorConv(inspector).reflect_table(t)
  File ".../alembic/autogenerate/compare/util.py", line 216, in _make_reflection_info
    table_key: self._return_from_cache(
  File ".../alembic/autogenerate/compare/util.py", line 113, in _return_from_cache
    return self._apply_reflectinfo_conv(individual)
  File ".../alembic/autogenerate/compare/util.py", line 181, in _apply_reflectinfo_conv
    if const["name"] is not None and not isinstance(
       ~~~~~^^^^^^^^
KeyError: 'name'

Reproduction steps: expected results

{'name': 'PK_singers', 'constrained_columns': ['id']}
[]

OS & version + platform

Linux

Python environment

Python 3.13

Python dependencies

sqlalchemy 2.1.5, alembic 1.20.0, google-cloud-spanner 3.71.0, sqlalchemy-spanner 1.20.1, Spanner emulator 1.5.58

Additional context

Spanner names primary key constraints PK_<table>, so get_multi_pk_constraint() can select tc.constraint_name along with the columns and return it as name.