Introduce SqlDefault for using expression as db_default

[abondar]

Resolves #2104

Introduce SqlDefault to pass expressions as db default
Change schema generator behaviour to match migrations (no default in db on default in Field)

AI Summary:
This pull request introduces support for raw SQL default expressions in database schemas via new SqlDefault and Now expressions, and clarifies the distinction between Python-only defaults and database-level defaults. It also updates documentation and tests to reflect these changes, and adjusts schema generation so that Python-only defaults (such as default=..., auto_now, and auto_now_add) do not emit DEFAULT clauses in generated schemas.

New features and API additions:

• Added SqlDefault and Now expressions for use with the db_default parameter, allowing raw SQL expressions (e.g., CURRENT_TIMESTAMP) as database defaults. Now() is a shorthand for SqlDefault("CURRENT_TIMESTAMP"). [1] [2] [3] [4] [5] [6]

Behavior changes and schema generation:

• Updated schema generation so that Field(default=...), auto_now, and auto_now_add no longer emit a DEFAULT clause in generate_schemas(). Only db_default is used for database-level defaults, aligning schema generation with migration behavior. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Documentation improvements:

• Updated documentation to clarify the difference between Python-only defaults and database-level defaults, and provided usage examples for db_default, SqlDefault, and Now. [1] [2] [3]

Testing:

• Added comprehensive tests for SqlDefault and Now, including construction, equality, hashing, schema generation, migration rendering, and serialization.

Examples and migrations:

• Updated example models and migrations to demonstrate the use of SqlDefault and Now in real project scenarios. [1] [2] [3]

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing featuer/sql-default (e61f6ea) with develop (e119288)}

[coveralls]

Pull Request Test Coverage Report for Build 21944692887

Details

• 47 of 61 (77.05%) changed or added relevant lines in 12 files are covered.
• 13 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 82.523%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tortoise/backends/oracle/schema_generator.py	0	1	0.0%
tortoise/migrations/expressions.py	0	1	0.0%
tortoise/backends/base/schema_generator.py	4	6	66.67%
tortoise/migrations/schema_editor/sqlite.py	0	10	0.0%

Files with Coverage Reduction	New Missed Lines	%
tortoise/converters.py	13	44.55%

Totals
Change from base Build 21925286894:	-0.1%
Covered Lines:	10578
Relevant Lines:	12351

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💛 - Coveralls