fix(Migrator): don't panic in AlterColumn/DropColumn when value is a table-name string

[h2zi]

What this PR does

The base GORM migrator accepts a table-name string as the value argument (RunWithValue sets stmt.Table and leaves stmt.Schema nil), and the other official dialects support this calling style. The sqlite driver dereferences stmt.Schema unconditionally in AlterColumn and DropColumn, so both crash with a nil pointer dereference:

db.Migrator().DropColumn("my_table", "my_column") // panic: invalid memory address

The fix

• DropColumn works fine without a model schema (the given column name is used as-is), so it now just skips the field lookup, mirroring the stmt.Schema != nil guard that HasColumn already has.
• AlterColumn cannot build the new column type without the model schema, so it returns a regular error instead of panicking.

Tests

TestMigratorStringTableName covers both paths: DropColumn with a table-name string succeeds and actually drops the column, AlterColumn returns an error instead of panicking.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes a nil-pointer panic in the SQLite GORM migrator when AlterColumn/DropColumn are called with a table-name string (i.e., stmt.Schema == nil), aligning behavior with GORM’s base migrator and other dialects.

Changes:

• Guard DropColumn’s schema field lookup behind stmt.Schema != nil to prevent panics when called with a table-name string.
• Make AlterColumn return a regular error (instead of panicking) when stmt.Schema == nil, since it requires schema to compute the new column type.
• Add a regression test covering both DropColumn (success) and AlterColumn (error) for the table-name string calling style.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
migrator.go	Prevents nil dereference in DropColumn and converts AlterColumn’s schema-nil panic into a regular error.
migrator_test.go	Adds regression coverage for table-name-string calls to DropColumn/AlterColumn.

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