Fix edge cases when inserting and deleting same object in one transac…

…tion (#71)

txn.go

@@ -744,6 +744,15 @@ func (txn *Txn) Changes() Changes {
 			// case it's different. Note that m is not a pointer so we are not
 			// modifying the txn.changeSet here - it's already a copy.
 			m.Before = mi.firstBefore
+
+			// Edge case - if the object was inserted and then eventually deleted in
+			// the same transaction, then the net affect on that key is a no-op. Don't
+			// emit a mutation with nil for before and after as it's meaningless and
+			// might violate expectations and cause a panic in code that assumes at
+			// least one must be set.
+			if m.Before == nil && m.After == nil {
+				continue
+			}
 			cs = append(cs, m)
 		}
 	}

txn_test.go

@@ -1711,6 +1711,27 @@ func TestTxn_Changes(t *testing.T) {
 				},
 			},
 		},
+		{
+			Name:            "insert and then delete same item in one txn",
+			TrackingEnabled: true,
+			Mutate: func(t *testing.T, tx *Txn) {
+				// Insert a new row
+				err := tx.Insert("one", basicRows[0])
+				if err != nil {
+					t.Fatalf("Err: %s", err)
+				}
+				// Delete the same row again
+				err = tx.Delete("one", basicRows[0])
+				if err != nil {
+					t.Fatalf("Err: %s", err)
+				}
+			},
+			WantChanges: Changes{
+				// Whole transaction is a big no-op. Initial implementation missed this
+				// edge case and emitted a mutation where both before and after were nil
+				// which violates expectations in caller.
+			},
+		},
 	}
 
 	for _, tc := range cases {