• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

add UniqueIndex attribute to Field strucrt

in some case,

1. defind a struct field and set uniqueIndex tag ony. and call AutoMigrate
2. in some case, you must call AutoMigrage more than once. (eg, restart your app or change struct), the db return unique = true, but field struct return unique = false.
3. it will be call AlterColumn. in some time(eg: partition table), it will be failure.

in schema/field.go
1.Add UniqueIndex field to Field's struct。
4.Resolve UniqueIndex value from StructField.

in migrator/migrator.go -> MigrateColumn
1.check unique, Check for the uniqueIndex and unique simultaneously

User Case Description

type Order struct{
   ID int                     `gorm:"uniqueIndex:uk@order"`
   BizTime time.Time `gorm:"uniqueIndex:uk@order"`  // it's partition field.
}

// db init
// init PartitionTable 
//..... first
if err := db.AutoMigrate(&Order{}); err != nil{
   painc(err)
}

//... db.AutoMigrate Again. 
if err := db.AutoMigrate(&Order{}); err != nil{
   painc(err)  // it will be painc.  becouse alter Partition Field BizTime
}