AutoMigrate recreates table with composite unique index even if the model has not changed

[tyr0chen]

GORM Playground Link

go-gorm/playground#660

Description

I'm using the GORM ORM framework for Golang and have encountered an issue with composite unique indexes. When using the following structure that includes a composite unique index, I noticed that the recreateTable process (which includes creating a temporary table, migrating data, deleting the old table, and renaming the table) is executed every time AutoMigrate is called, even if the model has not changed.

type Gateway struct {
  IP string    `gorm:"index:uniq_vip,unique"`
  UIN string `gorm:"index:uniq_vip,unique"`
}

However, there is no issue when using a single-field unique index like the following model:

type Gateway struct {
  IP string    `gorm:"index:uniq_vip,unique"`
  UIN string 
}

After investigating, I found that the alterColumn = true is set in the code at https://github.com/go-gorm/gorm/blob/v1.25.5/migrator/migrator.go#L500, because field.Unique is false while unique is true.

	// check unique
	if unique, ok := columnType.Unique(); ok && unique != field.Unique {
		// not primary key
		if !field.PrimaryKey {
			alterColumn = true
		}
	}

Further investigation revealed that the problematic code for the sqlite-driver is at https://github.com/go-gorm/sqlite/blob/v1.5.4/ddlmod.go#L165:

for _, column := range getAllColumns(matches[1]) {
				for idx, c := range result.columns {
					if c.NameValue.String == column {
						c.UniqueValue = sql.NullBool{Bool: strings.ToUpper(strings.Fields(str)[1]) == "UNIQUE", Valid: true}
						result.columns[idx] = c
					}
				}
			}

Since it is a composite unique index, getAllColumns(matches[1]) returns a length of 2, containing the column names IP and UIN. Neither of these individual fields is unique, but they form a composite unique index. The condition strings.Fields(str)[1]) == "UNIQUE" is not strict enough, causing each field to be set as unique.

I expect the check should be something like strings.Fields(str)[1]) == "UNIQUE" && len(getAllColumns(matches[1])) == 1, so that the unique attribute is only set for fields when there is only one field in the unique index, rather than setting unique for each field in the composite unique index. After modifying the condition and testing, the behavior is as expected.