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Chapter 5. Saving User Input: Testing the Database

Benefit of TDD: iterative style of development.

Our target: take to-do item input from user and sent it to server

5.1 Wiring Up Our Form to Send a POST Request

To let browser send a standard HTML POST request, steps needed:

Give <input> element a name= attribute
Wrap it in a <form> tag with method="POST"

several tricks used to debug unexpected failure in functional test:

Add print statements, to show, for example, what the current page text is.
Improve the error message to show more info about the current state.
Manually visit the site yourself.
Use time.sleep to pause the test during execution (Chosen to be used)

Reason of failure: CSRF token missed.
What's CSRF token:
- a unique, secret, unpredictable value that is generated by the server-side application and transmitted to the client in such a way that it is included in a subsequent HTTP request made by the client.
One of Django's CSRF protection methods: placing a little auto-generated token into each generated form, so the form can be identified as POST request coming from original site.

Embed CSRF token using `{% csrf_token %}

        <form method="POST">
            <input name="item_text" id="id_new_item" placeholder="Enter a to-do item" />
            {% csrf_token %}
        </form>

Result of running FT, page is layed with content but failed test, as server haven't wired to deal with POST request yet.

5.2 Processing a POST Request on the Server

Current home.html don't have action= attribute in <form>. Hence, it is submitting back to the / page (the same URL it was rendered from). It means the request is dealt by home_page function.

Add a test case into lists/tests.py

    def test_can_save_a_POST_request(self):
        response = self.client.post('/', data={'item_text': 'A new list item'})
        self.assertIn('A new list item', response.content.decode())

Test it will return error

AssertionError: 'A new list item' not found in '<html>\n    <head>\n        <title>To-Do lists</title>\n    </head>\n    <body>\n        <h1>Your To-Do list</h1>\n        <form method="POST">\n            <input name="item_text" id="id_new_item" placeholder="Enter a to-do item" />\n            <input type=\'hidden\' name=\'csrfmiddlewaretoken\' value=\'U3BKUu5Xl50FQMi8JNz5ZCCIkHKISJJ2QtVuLuZ3zdRmOWXh7hvwZfXIh76CQiN2\' />\n        </form>\n\n        <table id="id_list_table"></table>\n    </body>\n</html>'

We will pass variables from Python view code into HTML templates

5.3 Passing Python Variables to Be Rendered in the Template

Django's template syntax lets us include a python object in template using notation {{ ... }}
- renderer will display the object as a string

    <body>
        <h1>Your To-Do list</h1>
        <form method="POST">
            <input name="item_text" id="id_new_item" placeholder="Enter a to-do item" />
            {% csrf_token %}
        </form>

        <table id="id_list_table">
            <tr><td>{{ new_item_text }}</td></tr>
        </table>
    </body>

new_item_text is the variable name for the user input

Modify test case in lists/tests.py, assertTemplateUsed will check whether we are still using template

    def test_can_save_a_POST_request(self):
        response = self.client.post('/', data={'item_text': 'A new list item'})
        self.assertIn('A new list item', response.content.decode())
        self.assertTemplateUsed(response, 'home.html')

return error

    self.assertTemplateUsed(response, 'home.html')
  File "/home/jason/miniconda3/envs/python-tdd-book/lib/python3.6/site-packages/django/test/testcases.py", line 578, in assertTemplateUsed
    self.fail(msg_prefix + "No templates used to render the response")
AssertionError: No templates used to render the response

QUES: Why fail?

ANS: In lists/views.py, home_page function no longer return rendered page. When facing POST request, it return a HttpResponse

    if request.method == 'POST':
        return HttpResponse(request.POST['item_text'])
    return render(request, 'home.html')

QUES: How to fix it?

ANS: Modify home_page method to return rendered page.

QUES: How to use render function?

ANS: render function takes, (3rd arguments), a dictionary which maps template variable names to their values.

def home_page(request):
    return render(request=request, template_name='home.html', context={
        'new_item_text': request.POST['item_text'],
    })

5.3.1 An Unexpected Failure

Running test as shown above will return error

ERROR: test_uses_home_template (lists.tests.HomePageTest)
...
  File "/home/jason/miniconda3/envs/python-tdd-book/lib/python3.6/site-packages/django/utils/datastructures.py", line 85, in __getitem__
    raise MultiValueDictKeyError(repr(key))
django.utils.datastructures.MultiValueDictKeyError: "'item_text'"

Error is from test_uses_home_template test case. We broke the code path where there is no POST request; i.e. explain below

test_can_save_a_POST_request get POST request with '/', data={'item_text': 'A new list item'}
test_uses_home_template get POST request with only '/', while request.POST['item_text'] cannot find it.

QUES: How to fix it?

ANS: type of request.POST is 'django.http.request.QueryDict' is a dictionary. So dictionary method dict.get can be used to supply a default value ''

def home_page(request):
    return render(request=request, template_name='home.html', context={
        'new_item_text': request.POST.get('item_text', ''),
    })

...

5.4 Three Strikes and Refactor

Principle: Don't Repeat Yourself (DRY), or three strikes and refactor (事不过三); 同一个代码段重复三次以后就应该 remove duplication

QUES: What we do?
ANS: 将重复的 table = ... rows = ... self.assertIn ... 独立为 helper function (as shown below)

    def check_for_row_in_list_table(self, row_text):
        table = self.browser.find_element_by_id('id_list_table')
        rows = table.find_elements_by_tag_name('tr')
        self.assertIn(row_text, [row.text for row in rows])

5.5 The Django ORM and Our First Model

Object-Relational Mapper (ORM) is a layer of abstraction of data stored in database (tables, rows, and columns)
ORM let use work with database using OOP.
- Classes map to database tables
- Attributes map to columns
- Individual instance of the class is mapped to a row of data in database

Django comes with ORM. Using which, creating a new record in data is easy:

creating an object
assigning some attributes
calling .save() function.

Django also provide API for:

querying the database using class attributes of object .object
use simplest possible query .all() to retrieves all records for the table
returned object is a list-like object called 'QuerySet'
- Using 'QuerySet', we can extract individual objects

All these can be used in test case

class ItemModelTest(TestCase):

    def test_saving_and_retrieving_items(self):
        first_item = Item()
        first_item.text = 'The first (ever) list item'
        first_item.save()

        second_item = Item()
        second_item.text = 'Item the second'
        second_item.save()

        saved_items = Item.objects.all()
        self.assertEqual(saved_items.count(), 2)

        first_saved_item = saved_items[0]
        second_saved_item = saved_items[1]
        self.assertEqual(first_saved_item.text, 'The first (ever) list item')
        self.assertEqual(second_saved_item.text, 'Item the second')

5.5.1 Our First Database Migration

In Django, ORM's job is to model the database,
migration is a system that build the database. It has ability to add/remove tables and columns, based changes on models.py file. Like a version control system for the database.

Steps to update database:

python manage.py makemigrations to create migrations/000x_initials.py
Make migration via python manage.py migrate

5.5.2 The Test Gets Surprisingly Far

To pass the test, add class Item:

class Item(models.Model):
    text = models.TextField(default='')

Then migrate the database again, and then we can pass the test

5.6 Saving the POST to the Database

Modify the POST request test case that view will save a new item to the database instead of just passing to response.

    def test_can_save_a_POST_request(self):
        response = self.client.post('/', data={'item_text': 'A new list item'})

        self.assertEqual(Item.objects.count(), 1) # check that one new Item has been saved to the database
        new_item = Item.objects.first() # same as doing objects.all()[0]
        self.assertEqual(new_item.text, 'A new list item') # check that the item's text is correct
        
        self.assertIn('A new list item', response.content.decode())
        self.assertTemplateUsed(response, 'home.html')

First step to pass the test: modify home_page function to save request (including empty request just to '/')

def home_page(request):
    item = Item()
    item.text = request.POST.get('item_text', '')
    item.save()

    return render(request=request, template_name='home.html', context={
        'new_item_text': request.POST.get('item_text', ''),
    })

To-Do List to modify code (sometimes make sure code pass is ok, modify later):

Don't save blank items for every request (empty request)
Code small: POST test is too long?
Display multiple items in the table
Support more than one list

Solve the first todo list by creating a UT:

    def test_only_saves_items_when_necessary(self):
        self.client.get('/')
        self.assertEqual(Item.objects.count(), 0)

Running test will result AssertionError: 1 != 0

We can pass this UT via modifying home_page:

def home_page(request):
    if request.method == 'POST':
        new_item_text = request.POST['item_text'] # use new_item_text to either hold POST contents or empty string
        Item.objects.create(text=new_item_text) # .object.create is shortcut for creating a new Item, without needing to call .save()
    else:
        new_item_text = ''

    return render(request=request, template_name='home.html', context={
        'new_item_text': request.POST.get('item_text', ''),
    })

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