Where() should treat custom string types as SQL conditions

[dushaoshuai]

Describe the feature

Make Where() treat custom string types as SQL conditions like plain strings.

Motivation

Passing a value whose underlying type is string to Where causes GORM to treat it as a struct/model instead of a raw SQL snippet. The dry-run output shows the condition being rewritten to `some_table`. = "col1 IS NOT NULL" followed by the runtime error model value required.

package not_a_string_type_test

import (
	"context"
	"fmt"
	"testing"

	"gorm.io/gorm"
)

type notString string

func (ns notString) String() string {
	return string(ns)
}

const (
	col1 notString = "col1"
)

func Test_not_a_string_type(t *testing.T) {
	db := GetDB(context.Background()).
		Session(&gorm.Session{DryRun: true})

	var val map[string]any

	notStrCond := col1 + " IS NOT NULL"
	err := db.Table("some_table").
		Where(notStrCond).
		Find(&val).
		Error
	if err != nil {
		t.Error(err)
	}
	// [0.030ms] [rows:0] SELECT * FROM `some_table` WHERE `some_table`. = "col1 IS NOT NULL"
	//    not_a_string_type_test.go:35: model value required

	strCond := col1.String() + " IS NOT NULL"
	err = db.Table("some_table").
		Where(strCond).
		Find(&val).
		Error
	if err != nil {
		t.Fatal(err)
	}
	// [0.006ms] [rows:0] SELECT * FROM `some_table` WHERE col1 IS NOT NULL
}

I’m happy to contribute a PR for this change if it sounds reasonable.

Related Issues

nil.