Generic function for fetching different models in REST API - go

Question

I am writing a library in Go for using the Strava API. It's a simple API to expose the various objects (athlete, activity, and so on) that make up Strava's data. I am struggling to come up with a way that separates the mechanics of making a request so it can be reused to fetch the various different objects in the API. What I have so far:
type Model interface {
Url() *url.URL
Data() interface{} // pointer to location of unmarshaled response
}
// an activity (run, bike ride, etc)
type Activity struct {
Id int64 `json:"id"`
Name string `json:"name"`
Distance float64 `json:"distance"`
// ...
}
func (a *Activity) Url() *url.URL {
return url.Parse(fmt.Sprintf("https://www.strava.com/api/v3/activities/%d", a.Id))
}
func (a *Activity) Data() interface{} {
return a
}
// gear (shoes, bike, etc)
type Gear struct {
Id string `json:"id"`
Name string `json:"name"`
}
func (g *Gear) Url() *url.URL {
return url.Parse(fmt.Sprintf("https://www.strava.com/api/v3/gear/%s", g.Id))
}
func (g *Gear) Data() interface{} {
return g
}
// a page of activities
type ActivityPage struct {
AthleteId int64
PageNum int
Activities []Activity
}
func (p *ActivityPage) Url() *url.URL {
return url.Parse(fmt.Sprintf("https://www.strava.com/api/v3/athletes/%d/activities?page=%d&per_page=%d", p.AthleteId, p.PageNum, perPage))
}
func (p *ActivityPage) Data() interface{} {
return &p.Activities
}
type Client struct {
hc *http.Client
}
// error handling omitted
func (c *Client) fetch(m Model) error {
data, _ := c.fetchUrl(m.Url())
json.Unmarshal(data, m.Data())
return nil
}
func (c *Client) fetchUrl(u *url.URL) ([]byte, error) {
req := &http.Request{
// omit access token
Method: "GET",
URL: u,
}
resp, _ := c.hc.Do(req)
defer resp.Body.Close()
return ioutil.ReadAll(resp.Body)
}
The Data() pointer is needed because the GET /athlete/activities endpoint returns a list of Activitys, rather than a specific model. ActivityPage is therefore a bit of a hack — an object that carries the data needed to build the URL along with a place to put the results. For cases where a GET returns a model the Data() pointer is just the object.
With this code, I can do:
client := Client{}
activity := Activity{Id: 1234}
client.fetch(activity)
fmt.Print(activity.Name)
page := ActivityPage{AthleteId: 1, PageNum: 1}
client.fetch(page)
fmt.Print(len(page.Activities))
But this feels.. icky. I don't like partially constructing the object and passing it to fetch() to be finished off, or that fetch doesn't actually return anything except an error on failure. The Data() pointer is a hack.
AIUI, interfaces are a way to write code that can work with objects of different types, but I feel like I want the inverse — to have some code (a Fetch() method or something) that is inherited by all objects with a certain trait.
How can I make this cleaner? I realise this is kind of open-ended so I'm more than happy to refine what the exact question is as appropriate. Are there canonical examples of building a REST client in Go? (I haven't found anything compelling so far)

Answer 1

this is a standard workaround typical for go, you pass a pointer to data you want to be modified by reflection, standard lib is built like that, you can at most make method to accept pointer to data and url directly to be more verbose and not interface. It will make it at least match cleaner what will get modified just from looking at api calls for user like:
func (c *Client) fetch(url string, responceBuffer interface{}) error {
data, err := c.fetchUrl(url)
if err != nil {
return err
}
return json.Unmarshal(data, responceBuffer)
}