fix: type relation columns by their target, not by the relation

[imakecodes]

Closes #38.

Top of stack #61, based on #59. This one changes output for every user, which is why it is separated from the test work below it.

What was still broken

The symptom the reporter hit, a ref naming a column its table never declared, was fixed in #41. I reproduced their exact models and confirmed it. The rest of their report was not fixed:

prod_id foreign_key [not null]

foreign_key and one_to_one name the relation, which DBML already carries in the ref. As a column type they say nothing about what the column holds, and they never match the column they point at, so a reader cannot tell that Book.author_id and Author.id are the same thing.

The README has documented the correct behavior all along, with author_id bigint in its example. That is a type this package has never emitted.

The change

SchemaBuilder.get_field_type resolves a ForeignKey or OneToOneField to field.target_field. The synthesized many-to-many join table resolves both of its columns and its own primary key the same way, instead of hardcoding auto.

Reporter's models from #38, before and after:

Column	Before	After	Target
prod.TestA.prod_id	foreign_key	auto	prod.Prod.id is auto

Across the fixture app:

Column	Before	After
testapp.Book.author_id	foreign_key	big_auto
testapp.AuthorProfile.author_id	one_to_one	big_auto
testapp.Shipment.warehouse_id	foreign_key	auto
testapp.book_tags.book_id	auto	big_auto

Because the type now follows the target, a project that mixes DEFAULT_AUTO_FIELD values reports each relation correctly instead of uniformly. Nothing else about the schema moves: names, refs, indexes, notes and enums are untouched.

What holds it

Two invariants, both running over the whole fixture app rather than named columns:

• both ends of every relation must declare the same type
• no column may be typed by a relation kind

Plus unit tests on get_field_type and explicit assertions that the type tracks the target primary key.

Verification

53 tests, 99% coverage, make ci clean, green on Python 3.11 with Django 4.2 and Python 3.14 with Django 6.1. The reporter's original models now render prod_id auto against Prod.id auto.

README example corrected, and the rule documented in CLAUDE.md.

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