fix(changes): "left" side of report should be the previous path

The upper layers & the UI powered by the `Report` method expect that if the `leftRef` is empty, we should just use the `PreviousPath` of the `rightRef` as the `left` dataset to compare to.

This bug only occurs if the `left` side is empty, and currently all our `ChangeReport` consumers have filled in both versions to compare. There is now a test that ensures the behavior is correct.

changes/changes.go

@@ -386,14 +386,18 @@ func (svc *service) columnStatsDelta(left, right interface{}, lCol, rCol *tabula
 // Report computes the change report of two sources
 // This takes some assumptions - we work only with tabular data, with header rows and functional structure.json
 func (svc *service) Report(ctx context.Context, leftRef, rightRef dsref.Ref, loadSource string) (*ChangeReportResponse, error) {
-	leftDs, err := svc.loader.LoadDataset(ctx, leftRef, loadSource)
+	rightDs, err := svc.loader.LoadDataset(ctx, rightRef, loadSource)
 	if err != nil {
 		return nil, err
 	}
-	if rightRef.Path == "" {
-		rightRef.Path = leftDs.PreviousPath
+	if leftRef.Path == "" {
+		if rightDs.PreviousPath == "" {
+			return nil, fmt.Errorf("dataset has only one version")
+		}
+		leftRef.Path = rightDs.PreviousPath
 	}
-	rightDs, err := svc.loader.LoadDataset(ctx, rightRef, loadSource)
+
+	leftDs, err := svc.loader.LoadDataset(ctx, leftRef, loadSource)
 	if err != nil {
 		return nil, err
 	}

lib/changes_test.go

@@ -0,0 +1,37 @@
+package lib
+
+import (
+	"context"
+	"testing"
+
+	testcfg "github.com/qri-io/qri/config/test"
+	"github.com/qri-io/qri/event"
+	"github.com/qri-io/qri/p2p"
+	testrepo "github.com/qri-io/qri/repo/test"
+)
+
+func TestChanges(t *testing.T) {
+	ctx, done := context.WithCancel(context.Background())
+	defer done()
+
+	mr, err := testrepo.NewTestRepo()
+	if err != nil {
+		t.Fatalf("error allocating test repo: %s", err.Error())
+	}
+	node, err := p2p.NewQriNode(mr, testcfg.DefaultP2PForTesting(), event.NilBus, nil)
+	if err != nil {
+		t.Fatal(err.Error())
+	}
+	inst := NewInstanceFromConfigAndNode(ctx, testcfg.DefaultConfigForTesting(), node)
+
+	InitWorldBankDataset(ctx, t, inst)
+	commitref := Commit2WorldBank(ctx, t, inst)
+
+	// test ChangeReport with one param
+	p := &ChangeReportParams{
+		RightRefstr: commitref.Alias(),
+	}
+	if _, err := inst.Dataset().ChangeReport(ctx, p); err != nil {
+		t.Fatalf("change report error: %s", err)
+	}
+}