Django-tables2: Meta options are not inherited.

[applegrew]

Hi,

I have the following tables:-

    class BaseTable(tables.Table):
        select_column = tables.CheckBoxColumn(verbose_name='', empty_values=('NA',))

        class Meta:
            attrs = {'class' : 'cool_table'}

    class EditableTable(BaseTable):
        edit_column = EditIconColumn(verbose_name=_('Edit'))

        def set_editable(self, flag):
            if flag:
                self.base_columns['edit_column'].visible = True
            else:
                self.base_columns['edit_column'].visible = False   

    class RoomTable(EditableTable):
        number = tables.Column()
        speciality = tables.Column()
        photo = ImageViewColumn(accessor='room_type.photo')

When I render RoomTable the generated <table> does not have the class attribute. I am able to get that attribute only when I move (or copy) the Meta class from BaseTable to RoomTable.

Is this a bug or I am using it incorrectly?

[bradleyayers]

Thanks for the report this is a bug. It's now on the top of my todo list.

[applegrew]

Thanks.

[bradleyayers]

So I've almost finished implementing a solution to this, and then I remembered that for Meta inheritence I was following the Django style, i.e.

class Meta(Parent.Meta):
    ...

So now I'm starting to wonder whether it is actually a good idea to deviate from that. I suppose it's always been inconsitent anyway, in the first subclass of django_tables2.Table you _don't_ need to do class Meta(django_tables2.Tables):, but for any further subclasses you _do_.

I suppose you could say though that there is no django_tables2.Table.Meta API, and it only _starts_ at the first subclass. What's your feeling on this? Did you know how Django's model's Meta inheritence work and expect it to be different here? Or were you naive to Django's API and so you just tried what felt _right_?

[bradleyayers]

I've just re–read your original issue, and your issue seems to be that you're not even trying to change meta options in your subclass, you're just trying to subclass and add columns.

[applegrew]

Sorry I don't clearly understand what you are saying, but yes what I did was that felt right. Anyway, I am pretty new to Meta stuff so my approach may not be good.

[jrschneider]

Bradley,

Love the work on this project. I am running into the same issue as applegrew here, and your last comment matches my case perfectly.

### Table Definitions ###
# parent class 
import django_tables2 as tables

# generic parent class to handle table styling
class MyAppTable(tables.Table):
    class Meta:
        attrs = {'class': 'render-table'}


# subclass in a separate app
from myapp_helpers.tables import MyAppTable
from django_tables2.utils import A
import django_tables2 as tables

class ItemTable(MyAppTable):

    address_1 = tables.Column()
    address_2 = tables.Column()
    address_3 = tables.Column()
    city = tables.Column()
    state = tables.Column()
    zipcode = tables.Column()

    # need this to fix a bug with the meta class not passing through to subclasses in django-tables2
    class Meta(MyAppTable.Meta):
        pass


## Views ##

# class based view in separate app
from django_tables2 import SingleTableView

# Parent class to make pagination the same across views
class MyAppListView(SingleTableView):

    # default of 10 rows in every table on every page
    table_pagination = {"per_page": 10 }
    template_name = 'generic_list.html'


# subclass of view in local app
class ItemListView(MyAppListView):
    model = Item
    table_class = ItemTable

If the class Meta definition is not placed in ItemTable, the meta attributes set in the parent class are not sent through. I would like to have the same attrs sent to all of the tables in my apps (that eventually inherit from MyAppTable)

Defining the empty Meta class in ItemTable that inherits from the parent Meta class (MyAppTable) works, but seems to work against the DRY principle, even though it is only two lines and still meets my requirement of only needing to change the CSS class in one spot should I need to. Appreciate the work! Thank you.

[bradleyayers]

I know where you're coming from, and I tend to agree with you, in-fact I've partially written a patch to implement this, however this approach contradicts the approach Django takes with models. My current stance however is that having things work in a similar way to Django is important for having the library work as expected.

Do you think it's really worth breaking from Django + Python standard behaviour for this?

[jrschneider]

Having only developed using Django/Python for about a month, I don't think I'm qualified to make an argument for breaking from the norm. I suppose there are a few options though. You could:

a) Document the workaround as detailed above (if it isn't already, I might have missed it). It's really not that terrible.
b) Provide an attribute like 'inherit_meta = False' that can be overridden in subclasses to allow sub-classes to inherit the meta information. The default would be false to keep things working according to Django standards, but would allow more adventurous/lazy folks such as myself to eliminate the need for the repeated Meta class definitions. The only issue I see with that is maintaining an extra chunk of code for a workaround that could potentially introduce bugs (data present when not expected), etc.
c) Make this behavior the new default.

Even as I write this, I can already see scenarios in my head where template variables are checked based on the fact that inheritance doesn't occur, breakage occurs, and I spend hours poking around the code only to find out that it broke because I overrode the behavior manually. It also wouldn't match any of the existing examples.

Frankly, it probably isn't worth the effort. I would rather see an extra note about the meta inheritance 'gotcha' in the documentation than try to code in a workaround to generate atypical behavior.

[bradleyayers]

In regards to a), you do it like this:

class Parent(tables.Table):
    class Meta:
        foo = 1

class Child(Parent):
    class Meta(Parent.Meta):
        bar = 2

Child.Meta now has foo = 1 _and_ bar = 2

I tend to agree that this can be fixed with documentation.

[jrschneider]

Thanks. I actually had this up in my example based on applegrew's example.

[bradleyayers]

Doh, good point.

[SimonGreenhill]

One annoyance with the work-around is that you can't then override something in the Parent class:

class Parent(tables.Table):
    class Meta:
        attrs = {...}

class Child(Parent):
    class Meta(Parent.Meta):
         attrs = {'summary': 'A child table'}

raises a NameError with name 'attrs' is not defined. This, however, does seem to work, but is ugly:

class Child(Parent):
    class Meta(Parent.Meta):
        pass
     Meta.attrs['summary'] = 'A child table'

[paulnicholsen27]

Is there a solution to this bug yet? The above workaround doesn't work for me. http://stackoverflow.com/questions/21787938/inherit-and-modify-a-meta-class

[bradleyayers]

I don't see an obvious algorithm for merging together two arbitrary class Meta. Think about the cases for adding a key to attrs, removing a key from attrs, replacing attrs with a new dict, etc. Maybe I'm missing something but it's not obvious to me how you would do this in a sane way.

The behaviour should be the same as if you were subclasses models.Model classes.

[paulnicholsen27]

I'm not sure I follow what you're saying. I'm just trying to inherit the meta from the parent class and add a few columns. I've updated the code in the SO question above, which hopefully makes a bit more sense.

[pydolan]

@bradleyayers -- if I'm understanding your request correctly, you should be able to follow this StackOverflow post to overwrite Meta

[bradleyayers]

@pydolan Yes, check http://stackoverflow.com/questions/21787938/inherit-and-modify-a-meta-class for some other approaches too.