feat(sqlalchemy-bigquery): add JSON type support to BigQueryTypeCompiler

[geoHeil]

Description

BigQuery has natively supported the JSON type since 2022, but sqlalchemy-bigquery does not map it. This causes UnsupportedCompilationError when creating tables with sa.JSON() columns:

sqlalchemy.exc.UnsupportedCompilationError: Compiler <BigQueryTypeCompiler>
can't render element of type JSON

Changes

• base.py: Add visit_JSON to BigQueryTypeCompiler, returning "JSON"
• _types.py: Add "JSON": sqlalchemy.types.JSON to _type_map and export JSON
• __init__.py: Export JSON from the package
• tests/unit/test_compiler.py: Add unit test for JSON column DDL compilation

Testing

All 314 existing unit tests pass. New test verifies sa.JSON() compiles to JSON in CREATE TABLE DDL.

Context

Discovered while setting up Alembic migrations for Metaxy's BigQuery metadata store.

Fixes #16123

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces comprehensive support for the JSON data type within the sqlalchemy-bigquery dialect. By integrating sqlalchemy.types.JSON with BigQuery's native JSON type, it resolves UnsupportedCompilationError issues and enables users to define and manage JSON columns in their BigQuery schemas through SQLAlchemy, enhancing compatibility and functionality for applications leveraging BigQuery's modern data capabilities.

Highlights

• JSON Type Support: Added support for the sqlalchemy.types.JSON type in sqlalchemy-bigquery, allowing it to be correctly compiled to BigQuery's native JSON type.
• Compiler Implementation: Implemented the visit_JSON method within the BigQueryTypeCompiler to handle the compilation of JSON types.
• Type Mapping and Export: Mapped sqlalchemy.types.JSON to the BigQuery JSON type in the internal type map and exported the JSON type for public use.
• Unit Testing: Included a new unit test to verify that sqlalchemy.JSON() columns are correctly compiled into CREATE TABLE DDL statements for BigQuery.

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[gemini-code-assist]

Code Review

This pull request adds support for the JSON data type in sqlalchemy-bigquery, which is a valuable feature enhancement. The implementation is straightforward and correct, including adding the type mapping, the compiler visit method, and exporting the new type. A unit test is also included to verify the change. I have a minor suggestion for improving code maintainability by keeping type assignments sorted, consistent with other parts of the codebase.