Draft: fix JSONType[T] Pluck query error

[alingse]

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

What did this pull request do?

This is a fix MR for #200

and this cause a little breaking change in JSONType[T] usage

the bug

I ever post PR #174 , but I didn't have test enough for ALL gorm query API.
this exported Data T field worked well in Find, First method, and added in json_type_test.go.
and Data() T method is not so efficient than directly exported field , but it is wrong.

This issue #200 give a bad case query in Pluck

Gorm found the input ptr of value var field = &JSONType[*User] is a struct , and have an exported Field Data T
so it goto the this Struct's schema.Fields logic, and found this struct's Data T and checked field.DataType == ""

https://github.com/go-gorm/gorm/blob/532e9cf4ccce927249bcb102c09e4a9093aae4fe/schema/schema.go#L280-L283

so finally got an error

[error] invalid field found for struct gorm.io/datatypes.JSONType[*main.User]'s field Data: define a valid foreign key for relations or implement the Valuer/Scanner interface
find field in row  <nil> &{name 18}

the fix

I research the gorm and found it only collect ast.IsExported Field to scheme.Fields

and the JSONType[T] provide generic method Valuer and Scaner is only for a field not for another relation Table.

so I think make Data unexported is a better fix solution.

what about the breaking change

I this this breaking is small, and easy enough to change

result.Data ----> result.Data()

If same name is not ok. may be change to result.GetData()

Feel sorry to introduce some bug and cause a break change😢

User Case Description

		type Attribute struct {
			Sex   int
			Age   int
			Orgs  map[string]string
			Tags  []string
			Admin bool
			Role  string
		}
		type UserWithJSON struct {
			gorm.Model
			Name       string
			Attributes datatypes.JSONType[Attribute]
		}

		// Pluck
		var attr datatypes.JSONType[Attribute]
		DB.Model(&UserWithJSON{}).Limit(1).Order("id asc").Pluck("attributes", &attr)
		var attribute = attr.Data()
		AssertEqual(t, attribute.Age, 18)

Note for JSONSlice[T]

sadly db.Pluck still failed for JSONSlice[T] dest, because gorm will reflect the dest and process the Array and Slice

		reflectValueType := reflectValue.Type()
		switch reflectValueType.Kind() {
		case reflect.Array, reflect.Slice:
			reflectValueType = reflectValueType.Elem()
		}

so, I add a Note in README.md for JSONSlice[T]

[alingse]

cc @jinzhu I will add more test for all query for this JSONType[T] and also JSONSlice[T] later in weekend.

[alingse]

@jinzhu more test about query added for JSONType[T] and JSONSlice[T], the pr fix JSONType[T] Pluck query error (with an breaking API changes), but JSONSlice[T] Pluck query error still exists which might not be solved.