Make createcachetable use SchemaEditor for SQL generation

[Tim Graham]

The createcachetable management command ​generates its SQL manually. The generated SQL:

CREATE TABLE test_cache_table (
    cache_key STRING(255) NOT NULL PRIMARY KEY,
    value STRING(MAX) NOT NULL,
    expires TIMESTAMP NOT NULL
)

is incompatible with Google's Cloud Spanner databases, which requires a different PRIMARY KEY syntax:

CREATE TABLE test_cache_table (
    cache_key STRING(255) NOT NULL,
    value STRING(MAX) NOT NULL,
    expires TIMESTAMP NOT NULL
) PRIMARY KEY (cache_key)

This issue wouldn't happen if the management command used SchemaEditor.create_model() for SQL generation.

[Tim Graham]

draft patch

[Tim Graham]

The draft patch seems to work except for "Executing DDL statements while in a transaction on databases that can't perform a rollback is prohibited." (from 813805833aca60544f6b3d686c015039f6af5d1d) when running in non-dry-run mode.

[Simon Charette]

It's a shame that the schema editor cannot operate on model states and requires the creation of a temporary model here.

You'll want to use a throw away apps instead of a dummy app name, here's how the schema recorder does it

​https://github.com/django/django/blob/c1c361677d9400c8e2cdaddda0c16086bb358492/django/db/migrations/recorder.py#L30-L43

In order to address Executing DDL statements while in a transaction on databases that can't perform a rollback is prohibited. you should remove the outer atomic block and let SchemaEditor ​handle it if it's allowed by the underlying backend.

[Tim Graham]

​PR (tests not passing yet)