Golang database manager api concept, error with ty

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()))
}