This error is caused by the lack of distinction between Unique and UniqueIndex in schema.Filed and gorm.ColumnType.
A good way is to record UniqueIndex in them and compare them, and a not so good way is to re-compare them in AlterColumn (like go-gorm/postgres#157).

When AlterColumn, it'll run ALTER TABLE {table_name} MODIFY COLUMN {field} {fullDataType}. If fullDataType has UNIQUE, MySQL will always creates a new unique index.

So the solution is either checking if the unique index is already created before running the ALTER TABLE statement (your "not-so-good-way" I suppose?), or removing UNIQUE in fullDataType early (your "good-way" I suppose?). But I think the former way seems more reasonable, because fullDataType will clearly be used in other places like CreateTable.

This error is caused by the lack of distinction between Unique and UniqueIndex in schema.Filed and gorm.ColumnType. A good way is to record UniqueIndex in them and compare them, and a not so good way is to re-compare them in AlterColumn (like go-gorm/postgres#157).

When AlterColumn, it'll run ALTER TABLE {table_name} MODIFY COLUMN {field} {fullDataType}. If fullDataType has UNIQUE, MySQL will always creates a new unique index.

So the solution is either checking if the unique index is already created before running the ALTER TABLE statement (your "not-so-good-way" I suppose?), or removing UNIQUE in fullDataType early (your "good-way" I suppose?). But I think the former way seems more reasonable, because fullDataType will clearly be used in other places like CreateTable.

This error is caused by the lack of distinction between Unique and UniqueIndex in schema.Filed and gorm.ColumnType. A good way is to record UniqueIndex in them and compare them, and a not so good way is to re-compare them in AlterColumn (like go-gorm/postgres#157).

Because the migration needs to compare unique and unique index, they are also part of the migration. It is complicated for each driver to achieve such a comparison, and it is not conducive to expansion and maintenance, so the best way is to complete this repeated comparison by gorm operation, unless it cannot be done.
Of course, we also welcome submitting PRs for comparison in the driver, as it is good for quick code fixes

I have the same problem. Any update on this? If you need any help, I can do some coding.

#6381 (comment)