Added api/classifications endpoint, requiring a new query

internal/store/queries.go

@@ -75,7 +75,9 @@ SELECT t.* FROM opportunities t WHERE t.uuid = ?`
 
 // ListOpportunities is assembled by listOpportunitiesQuery as:
 //
-//	listOpportunitiesSelect [WHERE filter AND filter ...] listOpportunitiesOrderAndPage
+//	listOpportunitiesSelect
+//	[WHERE filter AND filter ...]
+//	listOpportunitiesOrderAndPage
 //
 // Each filterOpportunity* fragment takes exactly one "?" argument, except
 // filterOpportunityMark* fragments, which take none. A mark fragment is true
@@ -213,6 +215,17 @@ INSERT INTO opportunity_classifications (opportunity_uuid, classification_id)
 VALUES (?, ?)
 ON CONFLICT DO NOTHING`
 
+// selectClassificationsInUse reads every classification code that at least
+// one opportunity has, once each. c.* must map entirely onto
+// types.ClassificationCode.
+const selectClassificationsInUse = `
+SELECT c.* FROM classification_codes c
+WHERE EXISTS (
+	SELECT 1 FROM opportunity_classifications tc
+	WHERE tc.classification_id = c.id
+)
+ORDER BY c.code, c.id`
+
 // selectClassificationsForOpportunities reads the classifications of several
 // opportunities. "IN (?)" is expanded by sqlx.In, which must be followed by
 // Rebind. c.* must map entirely onto types.ClassificationCode.

internal/store/queries_test.go

@@ -100,6 +100,9 @@ var checks = map[string]queryCheck{
 	"insertOpportunityClassification": {
 		sql: insertOpportunityClassification,
 	},
+	"selectClassificationsInUse": {
+		sql: selectClassificationsInUse,
+	},
 	"selectClassificationsForOpportunities": {
 		sql: selectClassificationsForOpportunities,
 		in:  true,

internal/store/store.go

@@ -717,6 +717,21 @@ func (s *Store) ListReleases(
 	return releases, nil
 }
 
+// ListClassifications returns every classification code that at least one
+// stored opportunity has, ordered by code. A code no opportunity has any more
+// isn't listed. The result is never nil.
+func (s *Store) ListClassifications(
+	ctx context.Context,
+) ([]types.ClassificationCode, error) {
+	classifications := []types.ClassificationCode{}
+	err := s.db.SelectContext(ctx, &classifications, selectClassificationsInUse)
+	if err != nil {
+		return nil, fmt.Errorf("listing classifications: %w", err)
+	}
+
+	return classifications, nil
+}
+
 // LatestReleaseDate returns the date of the newest release stored for the
 // opportunities from source, or the zero time if none are stored, so a
 // fetcher can carry on from where it left off.

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)
+	}
+}