Reproduce issue where the underlying db will change the Scan type in `gorm`: https://github.com/jinzhu/gorm/issues/2276

gorm_diff_types_repro.go

package main

import (
	"database/sql/driver"
	"fmt"

	"github.com/jinzhu/gorm"
	_ "github.com/jinzhu/gorm/dialects/postgres"
	_ "github.com/jinzhu/gorm/dialects/sqlite"
)

func main() {
	dbSqlite, err := gorm.Open("sqlite3", "test.db")
	checkErr(err)
	dbPostgres, err := gorm.Open("postgres", "postgres://localhost:5432/gorm?sslmode=disable")
	checkErr(err)

	run("sqlite3 Scan into []uint8", dbSqlite)
	run("postgres Scan into []uint8", dbPostgres)
}

func run(label string, db *gorm.DB) {
	db.LogMode(true)
	instance := Table{CustomType: CustomType("hello world")}
	checkErr(db.AutoMigrate(instance).Error)
	checkErr(db.Save(&instance).Error)

	var retrieved Table
	err := db.First(&retrieved).Error

	if err != nil {
		fmt.Println(label, "failed", err)
	} else {
		fmt.Println(label, "success")
	}
}

type Table struct {
	gorm.Model
	CustomType CustomType
}

type CustomType string

// Value returns this instance serialized for database storage.
func (ct CustomType) Value() (driver.Value, error) {
	return string(ct), nil
}

// Scan reads the database value and returns an instance.
func (ct *CustomType) Scan(value interface{}) error {
	temp, ok := value.([]uint8)
	if !ok {
		return fmt.Errorf("Unable to convert %v of %T to CustomType", value, value)
	}

	*ct = CustomType(temp)
	return nil
}

func checkErr(err error) {
	if err != nil {
		panic(err)
	}
}

zoutput.txt

dimroc@Dimitris-MBP-2018: gomod $ go run gorm_diff_types_repro.go

(/Users/dimroc/workspace/garbage/gomod/gorm_diff_types_repro.go:26)
[2019-01-25 20:00:31]  [0.41ms]  INSERT INTO "tables" ("created_at","updated_at","deleted_at","custom_type") VALUES ('2019-01-25 20:00:31','2019-01-25 20:00:31',NULL,'hello world')
[1 rows affected or returned ]

(/Users/dimroc/workspace/garbage/gomod/gorm_diff_types_repro.go:29)
[2019-01-25 20:00:31]  [0.17ms]  SELECT * FROM "tables"  WHERE "tables"."deleted_at" IS NULL ORDER BY "tables"."id" ASC LIMIT 1
[1 rows affected or returned ]
sqlite3 Scan into []uint8 success

(/Users/dimroc/workspace/garbage/gomod/gorm_diff_types_repro.go:26)
[2019-01-25 20:00:31]  [0.70ms]  INSERT INTO "tables" ("created_at","updated_at","deleted_at","custom_type") VALUES ('2019-01-25 20:00:31','2019-01-25 20:00:31',NULL,'hello world') RETURNING "tables"."id"
[1 rows affected or returned ]

(/Users/dimroc/workspace/garbage/gomod/gorm_diff_types_repro.go:29)
[2019-01-25 20:00:31]  sql: Scan error on column index 4, name "custom_type": Unable to convert hello world of string to CustomType

(/Users/dimroc/workspace/garbage/gomod/gorm_diff_types_repro.go:29)
[2019-01-25 20:00:31]  [0.64ms]  SELECT * FROM "tables"  WHERE "tables"."deleted_at" IS NULL ORDER BY "tables"."id" ASC LIMIT 1
[1 rows affected or returned ]
postgres Scan into []uint8 failed sql: Scan error on column index 4, name "custom_type": Unable to convert hello world of string to CustomType
dimroc@Dimitris-MBP-2018: gomod $ vim gorm_diff_types_repro.go