5.0 step7 correction

Step 7 - Homework correction

Number "1"

Add the following method in library.book:

@fields.depends('description', 'summary')
def on_change_with_description(self):
    if self.description:
        return self.description
    if not self.summary:
        return ''
    return self.summary.split('.')[0]

The point here is to set the description field only if it is not yet set (hence the first if statement), and to extract some value from the summary field if it is set.

This pattern is useful to initialize fields from other fields, while still maintaining the possibility to completely change its value. The first if makes sure that a written down value will be kept even if the on_change is later triggered a second time.

Number "2"

Here we want to modify the value of the genre field depending on that of the editor field. This could be done by using a on_change_with on genre, or a on_change on editor. We are going to use the latter, because the editor field may later trigger other modifications (though there is no real "best" choice with the current elements):

@fields.depends('editor', 'genre')
def on_change_editor(self):
    if not self.editor:
        return
    if self.genre and self.genre not in self.editor.genres:
        self.genre = None
    if not self.genre and len(self.editor.genres) == 1:
        self.genre = self.editor.genres[0]

See how we can use the genres field of self.editor ? This is because the editor record exists "outside" the library.book model. If the user modifies it, the modification is saved as soon as the user validates, whereas the author field (if the book is created from the One2Many field on library.author) may be "currently being created", so there is the risk of accessing a "not yet set" field.

Number "3"

This is a default method in library.book:

@classmethod
def default_exemplaries(cls):
    return [{}]

We explained that the expected return values for One2Many field is a list of dictionaries. So here we create an empty dictionary (there aren't any value that we can set, empty will be enough).

Remember what we said about consistency? How about making sure the number_of_exemplaries field is up to date?

@fields.depends('exemplaries')
def on_change_with_number_of_exemplaries(self):
    return len(self.exemplaries or [])

You could say "Hey, that could be used as a getter", and you would be right. However, we wrote an optimized classmethod as getter, so it would be counter-productive to replace it with an instance method. Remember that you do not have to use a on_change_with as a getter if it does not suit your needs, it is just a nice possibility that may make your code simpler.