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I'm want to do data pagination with Google Cloud Platform Datastore and I've found an example on GCP's page (https://cloud.google.com/datastore/docs/concepts/queries) for doing it with Cursors and it works absolutely fine.

The example provided by Google has hardcoded the variables var tasks Task and var task Task and I would like to create a reusable function where I can pass the pointer to a struct array via a parameter typed interface and get this struct populated by this function. For example:

 type MyStruct1 struct 
 F1 string
 

 type MyStruct2 struct 
 F1 int
 

 func list(ctx context.Context, cursorStr string, data interface) 
 ...
 

 func main() 
 mystruct1 := make(MyStruct1, 0)
 list(ctx, "", &mystruct1)

 mystruct2 := make(MyStruct2, 0)
 list(ctx, "", &mystruct2)
 

My problem starts when I need to create in this function, a variable to store the record and then append it to the struct array passed as a pointer.

Example from Google

func SnippetIterator_Cursor() 
 ctx := context.Background()
 client, _ := datastore.NewClient(ctx, "my-proj")
 cursorStr := ""
 // [START datastore_cursor_paging]
 const pageSize = 5
 query := datastore.NewQuery("Tasks").Limit(pageSize)
 if cursorStr != "" 
 cursor, err := datastore.DecodeCursor(cursorStr)
 if err != nil 
 log.Fatalf("Bad cursor %q: %v", cursorStr, err)
 
 query = query.Start(cursor)
 

 // Read the tasks.
 var tasks Task << THIS IS WHAT I WANT TO BE GENERIC 
 var task Task. << THIS IS WHAT I WANT TO BE GENERIC 
 it := client.Run(ctx, query)
 _, err := it.Next(&task)
 for err == nil 
 tasks = append(tasks, task)
 _, err = it.Next(&task)
 
 if err != iterator.Done 
 log.Fatalf("Failed fetching results: %v", err)
 

 // Get the cursor for the next page of results.
 nextCursor, err := it.Cursor()
 // [END datastore_cursor_paging]
 _ = err // Check the error.
 _ = nextCursor // Use nextCursor.String as the next page's token.

My Generic function based on the code above

func list(ctx context.Context, kind string, data interface, pageSize int, cursorStr string) string 
 query := datastore.NewQuery(kind).Limit(pageSize)
 if cursorStr != "" 
 cursor, err := datastore.DecodeCursor(cursorStr)
 if err != nil 
 log.Fatalf("Bad cursor %q: %v", cursorStr, err)
 
 query = query.Start(cursor)
 
 it := query.Run(ctx)

 // HERE IS WHERE THE PROBLEM STARTS
 var vet interface
 var rec interface

 k, err := it.Next(rec)
 if err != nil 
 log.Println(err.Error())
 
 for err == nil 
 log.Println(k, rec) // PROBLEM: The key comes ok but rec comes nil
 vet = append(vet, rec)
 k, err = it.Next(rec)
 
 log.Println(vet) // PROBLEM: vet has only nils

 nextCursor, err := it.Cursor()
 if err != nil 
 log.Println(err.Error())
 

 data = vet
 return nextCursor.String()


func TestListFunc() 
 data := make(Tasks, 0)
 cursor := list(ctx, "Tasks", &data, 10, "")
 x, _ := json.MarshalIndent(data, " ", " ")
 log.Println(string(x))

PROBLEM: The Datastore iterator .Next() doesn't seem to store the record in a variable typed interface

Use the reflect package:

func list(ctx context.Context, kind string, dst interface, pageSize int, cursorStr string) string 
 client, _ := datastore.NewClient(ctx, "my-proj")
 query := datastore.NewQuery(kind).Limit(pageSize)
 if cursorStr != "" 
 cursor, err := datastore.DecodeCursor(cursorStr)
 if err != nil 
 log.Fatalf("Bad cursor %q: %v", cursorStr, err)
 
 query = query.Start(cursor)
 

 // Get reflect value for the result slice.
 results := reflect.ValueOf(dst).Elem()

 // Allocate new value of the slice element type. 
 // resultp is pointer to that value.
 resultp := reflect.New(results.Type().Elem())

 it := client.Run(ctx, query)
 _, err := it.Next(resultp.Interface())
 for err == nil 
 // Append last value to results
 results.Set(reflect.Append(results, resultp.Elem())

 _, err = it.Next(resultp.Interface())
 
 if err != iterator.Done 
 log.Fatalf("Failed fetching results: %v", err)
 

 // Get the cursor for the next page of results.
 nextCursor, err := it.Cursor()
 // [END datastore_cursor_paging]
 _ = err // Check the error.
 _ = nextCursor // Use nextCursor.String as the next page's token.

Call the function with a pointer to the destination slice:

var data Tasks
cursor := list(ctx, "Tasks", &data, 10, "")