省市区三级联动的应用非常广泛，涉及到用户档案、收货地址什么的都需要用的，Go-Admin通过Field的FieldOptions、FieldOnChooseAjax、FieldOptionInitFn三个方法来完成三级联动。

一、准备基础数据方法[gorm]

//加载省
func GetMercProvOption() []types.FieldOption{
    var results []types.FieldOption
    var list []GopayMercProv

    PayDB.Order("prov_name").Limit(50).Find(&list)

    for _, item := range list {
        results = append(results, types.FieldOption{Text: item.ProvName, Value: item.ProvCode})
    }
    return  results
}

//城市
func GetMercCityOptionByProv(prov string) []selection.Option{
    var results []selection.Option
    var list []GopayMercCity

    PayDB.Where("prov_code=?",prov).Order("city_name").Limit(50).Find(&list)

    for _, item := range list {
        results = append(results, selection.Option{ Text : item.CityName, ID: item.CityCode})
    }
    return  results
}

func GetMercCityFieldOptionByProv(prov ,city string ) []types.FieldOption{
    var results []types.FieldOption
    var list []GopayMercCity

    PayDB.Where("prov_code=?",prov).Order("city_name").Limit(50).Find(&list)

    for _, item := range list {
        results = append(results, types.FieldOption{ Text : item.CityName, Value: item.CityCode , Selected: item.CityCode==city })
    }
    return  results
}

//区
func GetMercCountyOptionByCity(city string) []selection.Option{
    var results []selection.Option
    var list []GopayMercCounty

    PayDB.Where("city_code=?",city).Order("county_name").Limit(50).Find(&list)

    for _, item := range list {
        results = append(results, selection.Option{ Text : item.CountyName, ID: item.CountyCode})
    }
    return  results
}

func GetMercCountyFieldOptionByCity(city, county string) []types.FieldOption{
    var results []types.FieldOption
    var list []GopayMercCounty

    PayDB.Where("city_code=?",city).Order("county_name").Limit(50).Find(&list)

    for _, item := range list {
        results = append(results, types.FieldOption{ Text : item.CountyName, Value: item.CountyCode, Selected: item.CountyCode == county })
    }
    return  results
}

二、省市区下拉框

    formList.AddField("归属省", "merc_prov", db.Varchar, form.SelectSingle).
        FieldOptions(entitys.GetMercProvOption()).
        FieldOnChooseAjax("merc_city", "/choose/prov", func(ctx *context.Context) (bool, string, interface{}) {
            prov := ctx.FormValue("value")
            return true, "ok", entitys.GetMercCityOptionByProv(prov)
        })

    formList.AddField("归属市", "merc_city", db.Varchar, form.SelectSingle).
        FieldOnChooseAjax("merc_county", "/choose/city", func(ctx *context.Context) (bool, string, interface{}) {
            city := ctx.FormValue("value")
            return true, "ok", entitys.GetMercCountyOptionByCity(city)
        }).
        FieldOptionInitFn(func(val types.FieldModel) types.FieldOptions {
            var prov = val.Row["merc_prov"].(string)
        var city = val.Row["merc_city"].(string)
            return entitys.GetMercCityFieldOptionByProv(prov,city)
        })

    formList.AddField("归属县区", "merc_county", db.Varchar, form.SelectSingle).
        FieldOptionInitFn(func(val types.FieldModel) types.FieldOptions {
            var city = val.Row["merc_city"].(string)
            var county = val.Row["merc_county"].(string)
            return entitys.GetMercCountyFieldOptionByCity(city,county)
        })