Golang database manager api concept, error with type assertion - sql

Question

The base concept creating a Database Manager API for getting data through an API. I am using the GORM for getting data of the instances of the strcuts. So there is 300-400 struct which represents the tables.
type Users struct {
ID int64
Name string
}
type Categories struct {
ID int64
Category string
}
The next step I implement a function which is return the correct instance of the struct by table name, what I get through the API endpoint param.
func GetModel(model string) interface{} {
switch model {
case "users":
return Users{}
case "categories"
return Categories{}
}
return false
}
After there is an operations struct where the only one field is the DB. There is methods, for example the GetLast() where I want to use the GORM db.Last(&users) function.
func (o Operations) GetLast(model string) interface{} {
modelStruct := GetModel(model)
.
.
.
return o.DB.Last(&modelStruct)
}
There is points so this is what I don't know. The current solution is not working because in this case it is an interface{} I need make a type assertion more info in this question. The type assertion is looks like:
func (o Operations) GetLast(model string) interface{} {
modelStruct := GetModel(model)
.
test := modelStruct.(Users)
.
return o.DB.Last(&test)
}
This solution working, but in this case I lost the modularity. I try using the reflect.TypeOf(modelStruct), but it is also not working because the result of the reflect.TypeOf is a reflect.Type, with is not a golang type.

Answer 1

Basically I solved the problem, for getting the model as a pointer, and after I return it back as a json file.
So my model is the following:
var Models = map[string]interface{}{
"users": new(Users),
"categories": new(Categories),
}
And it is return back a new model by table type. what I can use for gorm First() function. Then json Marshal it, and return.
func (o Operation) First(model string, query url.Values) string {
modelStruct := Models[model]
db := o.DB
db.First(modelStruct)
response, _ := json.Marshal(modelStruct)
clear(modelStruct)
return string(response)
}
Before the return I clear the model pointer because the First() function store callbacks for the latest queries.
func clear(v interface{}) {
p := reflect.ValueOf(v).Elem()
p.Set(reflect.Zero(p.Type()))
}