Add field tag to ignore migration

[yrong]

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

What did this pull request do?

User Case Description

import (
	"github.com/shopspring/decimal"
       ...
)

type ChainExtrinsic struct {
Fee                decimal.Decimal `json:"fee" sql:"type:decimal(30,0);"`//v1
...
}
type ChainExtrinsic struct {
Fee                decimal.Decimal `json:"fee" gorm:"type:decimal(30,0);"`//v2
...
}
db.AutoMigrate()

/*Actually there is no field definition change between v1 and v2.
for mysql migration from gorm v1 to v2 the unnessary alter column ddl as following will be generated
the worst thing is after migrating to v2, will still generate ddl each time when run automigrate
---ALTER TABLE `chain_extrinsics` MODIFY COLUMN `fee` decimal(30,0)
*/

we can avoid by add field tag as following:

type ChainExtrinsic struct {
Fee                decimal.Decimal `json:"fee" gorm:"type:decimal(30,0);ignoreMigration"`
...
}

[jinzhu]

Hello @yrong

For you example, I would suggest you add a precision tag like:

type ChainExtrinsic struct {
  Fee                decimal.Decimal `json:"fee" gorm:"type:decimal(30,0);precision:30"`
  ...
}

but the skip migration feature sounds interesting, maybe we could support it like following, would you mind to make the change?

type ChainExtrinsic struct {
  Fee                decimal.Decimal `json:"fee" gorm:"type:decimal(30,0);-:migration"`
  ...
}

[yrong]

@jinzhu
Thanks for the clarifaction about precision tag and will take a try. For the skip migration I think maybe there is some confliction with the exist Field-Level Permission Definition which is essentially different and better not mixed together

Name string gorm:"-" // ignore this field when write and read with struct

[yrong]

@jinzhu
Btw, I add one minor fix 4a1096b for model definition as following which works in gorm v1 but failed in v2, maybe something related to #3363

Address string gorm:"default: null;size:100" json:"address" //there is a leading space before null

[jinzhu]

@jinzhu
Thanks for the clarifaction about precision tag and will take a try. For the skip migration I think maybe there is some confliction with the exist Field-Level Permission Definition which is essentially different and better not mixed together

Name string gorm:"-" // ignore this field when write and read with struct

I think it would be good to group all field permission related things into similar tags, so users can get them in one place.

Maybe we could have different permission level like the ->, <- one, - means disable read/write by default, -:all means disable read/write & migration, -:migration means only disable the migration?

[yrong]

@jinzhu
some refactor per your request, pls check if anything missing

[jinzhu]

Thank you for your PR

[birdycn]

I want to Add field tag to ignore migration,too

What should I do now