Gorm model只有一个字段并设置为unique,使用AutoMigrate，第二次启动服务Migrate会报错

model只有一个字段并设置为unique,使用AutoMigrate，第二次启动服务Migrate会报错

type Labels struct {
    Label string `gorm:"column:label;size:50;unique;not null;"`
}

...
if err = DB.AutoMigrate(Labels{}); err != nil {
       panic(err)
}

2024/08/22 16:51:33  Error 1061 (42000): Duplicate key name 'uni_labels_label'
[22.362ms] [rows:0] ALTER TABLE `labels` ADD CONSTRAINT `uni_labels_label` UNIQUE (`label`)
panic: Error 1061 (42000): Duplicate key name 'uni_labels_label'

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Comment From: github-actions[bot]

Comment From: xxxVitoxxx

In your example, the column label is not formally defined as a primary key, but the combination of UNIQUE and NOT NULL constraints leads to similar enforcement of uniqueness and non-nullability, effectively treating it as a primary key in practice.

In fact, the unique key has been created, but the primary key has not.

+--------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table  | Non_unique | Key_name         | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+--------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| labels |          0 | uni_labels_label |            1 | label       | A         |           0 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+--------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+

Although gorm has created a unique key, the label field is regarded as the primary key. As a result, in gorm, thePrimaryKeyValue of the label field's ColumnType is true, and its UniqueValue is false.

func main() {
    ct, _ := DB.Migrator().ColumnTypes(&Labels{})
    fmt.Printf("%+v\n", ct)
}
// output
// [{
//     SQLColumnType:0x14000138690 
//     NameValue:{String:label Valid:true} 
//     DataTypeValue:{String:varchar Valid:true} 
//     ColumnTypeValue:{String:varchar(50) Valid:true} 
//     PrimaryKeyValue:{Bool:true Valid:true} 
//     UniqueValue:{Bool:false Valid:true} 
//     AutoIncrementValue:{Bool:false Valid:false} 
//     LengthValue:{Int64:50 Valid:true} 
//     DecimalSizeValue:{Int64:0 Valid:false} 
//     ScaleValue:{Int64:0 Valid:false} 
//     NullableValue:{Bool:false Valid:true} 
//     ScanTypeValue:<nil> 
//     CommentValue:{String: Valid:true} 
//     DefaultValueValue:{String: Valid:false}
// }]

If you execute AutoMigrate again, a Duplicate key name error will occur because gorm determines that the unique key has not been created based on the ColumnType.

mysql/migrator.go Line 107 in v1.5.4

unique, ok := columnType.Unique()
// --snip--
if unique {
    // --snip--
} else {
    if field.Unique {
        if err := execTx.Migrator().CreateConstraint(value, constraint); err != nil {
        return err
    }
    // --snip--
}

should we avoid this error in this case?
@jinzhu

Comment From: github-actions[bot]