Golang Print structure always report undefined

May 15, 2022

Here is the structure I defined – generated from a json to Go struct tool:

type NYTimesNews struct {
    Data struct {
        LegacyCollection struct {
            CollectionsPage struct {
                Stream struct {
                    Edges []struct {
                        Node struct {
                            FirstPublished string `json:"firstPublished"`
                            Headline       struct {
                                Default string `json:"default"`
                            } `json:"headline"`
                            Summary string `json:"summary"`
                            URL     string `json:"url"`
                        } `json:"node"`
                    } `json:"edges"`
                } `json:"stream"`
            } `json:"collectionsPage"`
        } `json:"legacyCollection"`
    } `json:"data"`
}

When I iterate response from my request at the layer of Nodes ,everything works right and can be print out, below are the code of print out all Nodes which is in Edges array

  for i, Node:= range data.Data.LegacyCollection.CollectionsPage.Stream.Edges{
      // fmt.Printf("[%d] \n %s \n", i, Node)
      fmt.Printf("[%d] \n %s \n %s\n", i, reflect.TypeOf(Node),Node)
  }

Output

[0] 
 struct { Node struct { FirstPublished string "json:\"firstPublished\""; Headline struct { Default string "json:\"default\"" } "json:\"headline\""; Summary string "json:\"summary\""; URL string "json:\"url\"" } "json:\"node\"" } 
 {{2022-05-14T21:17:13.000Z {Turkey Offers to Evacuate Mariupol Fighters Despite Disagreements} Turkey has had a ship waiting for weeks in Istanbul to evacuate those remaining in the Azovstal steel plant, but Ukraine and Russia have not agreed to a plan, a Turkish official said. https://www.nytimes.com/2022/05/14/world/europe/azovstal-evacuation-turkey.html}}
[1] 
 struct { Node struct { FirstPublished string "json:\"firstPublished\""; Headline struct { Default string "json:\"default\"" } "json:\"headline\""; Summary string "json:\"summary\""; URL string "json:\"url\"" } "json:\"node\"" } 
 {{2022-05-14T16:26:03.000Z {Catalan Pop? Corsican Rock? It’s Europe’s Other Song Contest.} The Liet International, a competition for minority and regional languages, lacks the glitz of Eurovision. But its organizers say it helps keep endangered tongues alive. https://www.nytimes.com/2022/05/14/arts/music/minority-languages-song-contest.html}}

But when I tried to access single fields in Node structure error occured

Code after modify:

  for i, Node:= range data.Data.LegacyCollection.CollectionsPage.Stream.Edges{
    fmt.Printf("[%d] \n %s \n %s -------- \n\n", i, reflect.TypeOf(Node), Node.FirstPublished)
    break
  }

Output

./getdata-url-nytimes.go:92:80: Node.FirstPublished undefined (type struct{Node struct{FirstPublished string "json:"firstPublished""; Headline struct{Default string "json:"default""} "json:"headline""; Summary string "json:"summary""; URL string "json:"url""} "json:"node""} has no field or method FirstPublished)

Anything goes wrong when I use ‘doc fieldname’ to print a field in a Go structure ?

>Solution :

You named loop variable Node but what it really contains is a struct of type Edge. .Node is property of each Edge in the collection you iterate over. You need to access it through edge’s .Node field like this instead:

  for i, edge:= range data.Data.LegacyCollection.CollectionsPage.Stream.Edges{
    fmt.Printf("[%d] \n %s \n %s -------- \n\n", i, reflect.TypeOf(edge.Node), edge.Node.FirstPublished)
    break
  }