Added api/classifications endpoint, requiring a new query

internal/store/store_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -902,3 +903,44 @@ func TestSaveResultNewOpportunity(t *testing.T) {
 		)
 	}
 }
+
+// ListClassifications lists the codes in use, once each and sorted, and not
+// a code no opportunity has any more.
+func TestListClassifications(t *testing.T) {
+	s := openStore(t)
+	ctx := context.Background()
+
+	got, err := s.ListClassifications(ctx)
+	if err != nil || got == nil || len(got) != 0 {
+		t.Fatalf("empty store: %v, %v; want an empty, non-nil list", got, err)
+	}
+
+	mustSave(t, s, opportunity("ocds-1", "First", 1, "r1"))
+	mustSave(t, s, opportunity("ocds-2", "Second", 1, "r1"))
+	codes := func() []string {
+		t.Helper()
+		got, err := s.ListClassifications(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+		var listed []string
+		for _, classification := range got {
+			listed = append(listed, classification.Code)
+		}
+		return listed
+	}
+	if got := strings.Join(codes(), ","); got != "03000000,72000000" {
+		t.Fatalf("codes = %s; want 03000000,72000000", got)
+	}
+
+	// Newer releases replace both opportunities' classifications with one
+	// code; the other is still stored, but no longer listed.
+	for _, uuid := range []string{"ocds-1", "ocds-2"} {
+		newer := opportunity(uuid, "Newer", 2, "r2")
+		newer.Classifications = newer.Classifications[1:]
+		mustSave(t, s, newer)
+	}
+	if got := strings.Join(codes(), ","); got != "03000000" {
+		t.Errorf("after the change: codes = %s; want 03000000", got)
+	}
+}