if err != nil {
		t.Fatalf("Should open db success, but got %v", err)
	}

	tx := db.Begin()
	user := *GetUser("transaction", Config{})

	if err = tx.Save(&user).Error; err != nil {
		t.Fatalf("No error should raise, but got %v", err)
	}

	if err = tx.First(&User{}, "name = ?", "transaction").Error; err != nil {
		t.Fatalf("Should find saved record, but got %v", err)
	}

In this case, you could want to document how that external API *should* look like. What *path operation* it should have, what body it should expect, what response it should return, etc.

## An app with callbacks

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

    int index = aValidIndex();
    E initialValue = getList().get(index);
    assertEquals(
        "set(i, x) should return the old element at position i.",
        initialValue,
        getList().set(index, newValue));
    assertEquals("After set(i, x), get(i) should return x", newValue, getList().get(index));
    assertEquals("set() should not change the size of a list.", getNumElements(), getList().size());
  }

  @ListFeature.Require(SUPPORTS_SET)

    assertEquals("Should be able to peek() at first element", "A", peekingIterator.peek());
    assertEquals(
        "Should be able to peek() first element multiple times", "A", peekingIterator.peek());
    assertEquals(
        "next() should still return first element after peeking", "A", peekingIterator.next());

    assertEquals("Should be able to peek() at middle element", "B", peekingIterator.peek());

		t.Errorf(fmt.Sprintf("Count should work, but got err %v", err))
	}
	if count1 != 1 {
		t.Errorf("Count with group should be 1, but got count: %v", count1)
	}

	var count2 int64
	if err := DB.Model(&Company{}).Where("name in ?", []string{"company_count_group_b", "company_count_group_c"}).Group("name").Count(&count2).Error; err != nil {
		t.Errorf(fmt.Sprintf("Count should work, but got err %v", err))
	}
	if count2 != 2 {

    assertEquals("Cause should be available.", exception, handler.exception);
    assertEquals("EventBus should be available.", eventBus, handler.context.getEventBus());
    assertEquals("Event should be available.", EVENT, handler.context.getEvent());
    assertEquals("Subscriber should be available.", subscriber, handler.context.getSubscriber());
    assertEquals(
        "Method should be available.",

    assertEquals("Cause should be available.", exception, handler.exception);
    assertEquals("EventBus should be available.", eventBus, handler.context.getEventBus());
    assertEquals("Event should be available.", EVENT, handler.context.getEvent());
    assertEquals("Subscriber should be available.", subscriber, handler.context.getSubscriber());
    assertEquals(
        "Method should be available.",

  @CollectionSize.Require(absent = ZERO)
  public void testRemove_present() {
    int initialSize = collection.size();
    assertTrue("remove(present) should return true", collection.remove(e0()));
    assertEquals(
        "remove(present) should decrease a collection's size by one.",
        initialSize - 1,
        collection.size());
    expectMissing(e0());
  }

    assertEquals("Should be able to peek() at first element", "A", peekingIterator.peek());
    assertEquals(
        "Should be able to peek() first element multiple times", "A", peekingIterator.peek());
    assertEquals(
        "next() should still return first element after peeking", "A", peekingIterator.next());

    assertEquals("Should be able to peek() at middle element", "B", peekingIterator.peek());

  // terminating state of endpoints. This condition should be set to true for
  // a ready endpoint that is terminating. If nil, consumers should defer to
  // the ready condition.
  // +optional
  optional bool serving = 2;

  // terminating indicates that this endpoint is terminating. A nil value
  // indicates an unknown state. Consumers should interpret this unknown state
  // to mean that the endpoint is not terminating.