Resolved #2108 and some other backend specific issues

AI summary:
This pull request introduces several improvements and new features to the migrations and schema editing system, with a focus on enhanced cross-database compatibility, new constraint management, and improved test coverage. The most significant changes include the addition of a dialect-aware random hex default generator, support for named unique constraints in migrations, enhancements to schema editors for MySQL and MSSQL, and comprehensive tests for unique constraint handling.

Major Features and Enhancements

• Added a new RandomHex dialect-aware SqlDefault subclass to generate random hex strings as default values across all supported backends, replacing backend-specific SQL expressions. (RandomHex) is now used in both models and migrations for fields like tracking_id. [1] [2] [3] [4] [5]
• Introduced support for Meta.constraints on models, enabling named UniqueConstraint objects to be captured by the migration autodetector. This allows for automatic generation of AddConstraint and RemoveConstraint operations in migrations. [1] [2] [3]

Schema Editor and Backend Improvements

• Enhanced MySQL and MSSQL schema editors:
  • MySQL: _alter_field now uses MODIFY COLUMN for NULL/NOT NULL changes and backtick-quoted templates.
  • MSSQL: _alter_field uses ALTER COLUMN for nullability, manages named default constraints, uses bracket-quoted templates, and downgrades self-referencing FK CASCADE to NO ACTION.

Migration and Model Changes

• Updated migrations to use dialect-specific SQL for computed fields and switched from RunSQL to RunPython for better backend compatibility. [1] [2] [3]
• Adjusted the order of model creation in migrations for consistency and correctness. [1] [2]

Testing Improvements

• Added comprehensive tests for unique constraint handling, including:
  • Inline unique constraints in CREATE TABLE statements.
  • Proper dropping of unique constraints when altering fields from unique to non-unique.
  • Backend-specific introspection and fallback behavior for constraint names in PostgreSQL and MySQL. [1] [2] [3]

Documentation

• Updated the CHANGELOG.rst to document all new features, backend improvements, and bug fixes included in this release.