fix(migrations): emit Meta.constraints in CREATE TABLE (#2272)

[jagadeepmamidi]

Named UniqueConstraint and CheckConstraint were stored on CreateModel but never applied when creating tables, and MySQL add_constraint ignored custom names.

Description

Meta.constraints (UniqueConstraint / CheckConstraint) are now emitted in CREATE TABLE for both tortoise migrate and generate_schemas.

Previously makemigrations stored them on CreateModel, but table creation only applied unique_together, so a new model never got AddConstraint and named uniques like unique_like never appeared. MySQL add_constraint also ignored name and generated uidx_* instead.

• Named unique and check constraints are inlined in CREATE TABLE
• Partial unique constraints (condition) become CREATE UNIQUE INDEX ... WHERE on PostgreSQL
• A UniqueConstraint whose resolved columns already match unique_together is skipped
• MySQL add_constraint honors constraint.name when set

Motivation and Context

Fixes #2272

Reporter used tortoise makemigrations / tortoise migrate on MySQL 8.4 with:

class Meta:
    constraints = [
        UniqueConstraint(fields=("types", "user_id", "art_id"), name="unique_like"),
    ]

Expected a unique index named unique_like; nothing was created, with no error.

How Has This Been Tested?

Added regression tests in tests/migrations/test_schema_editor_sql.py and tests/migrations/test_schema_editor_constraints.py covering:

• CreateModel SQL for named unique + check constraints (ANSI and MySQL)
• skip when UniqueConstraint duplicates unique_together (including FK field vs organization_id columns)
• PostgreSQL partial unique index on create
• generate_schemas including Meta.constraints
• MySQL add_constraint using the custom name

Ran:

uv run pytest tests/migrations/test_schema_editor_sql.py tests/migrations/test_schema_editor_constraints.py

(52 passed)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[seejah]

Great efficient work 🏅

[jagadeepmamidi]

@seejah please let me know if I can check any other issues too (happy to help)

[codspeed]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing jagadeepmamidi:fix/2272-meta-constraints-create-table (2045bec) with develop (ffc946e)}

[waketzheng]

For _partial_unique_index_sqls(): the base class could just return [], and the
actual logic should move to an override in
tortoise/backends/base_postgres/schema_generator.py. Keeping self.DIALECT != "postgres"
in the base generator couples it to a specific dialect — new dialects should extend via
override, not by adding string checks here.

With that change, the MySQL override of _partial_unique_index_sqls() also becomes
unnecessary, since it would inherit the base return [] and correctly produce nothing.

[jagadeepmamidi]

Thanks for the feedback. I moved partial unique-index generation out of the base schema classes and into the PostgreSQL-specific implementations. The base classes now
return an empty list, so dialect-specific behavior is handled through backend overrides instead of checking for postgres in the base implementation.

I also added regression coverage for PostgreSQL generate_schemas() partial unique indexes. The branch was updated with the latest develop, and the targeted migration