What is the best way to deal with NULL column values

[Grizzle]

In my application one of the columns is an attribute of a related model as seen below.

columns = ['location.room.site.code', 'row', 'code', 'name', 'owner']

In normal circumstances it works fine because most of the time the model I'm using for this datatable has a related location model.

The relation isn't required so when I first create the object without linking it to a location my datatable fails to render with the following error in the frontend.

DataTables warning: table id=datatable - Ajax error. For more information about this error, please see http://datatables.net/tn/7

and the following error on the Django end

django.db.models.fields.related_descriptors.RelatedObjectDoesNotExist: Rack has no location.

So I am thinking I need to override the get_columns() method and put in some way to handle this scenario. I'm fine with returning an empty/null value as there are cases where this will be a legitimate behaviour.

Am I correct in thinking this or is there a better way to do this?

[Maciej Wisniowski]

I think you can override render_column method, something like below (code not tested):

def render_column(self, row, column):
    if column == 'location.room.site.code':
        # do your stuff here, eg.
        try:
             return row.location.room.site.code
         except Location.DoesNoteExists:  # assuming you have Location model
             return '-'
    return super().render_column(row, column)

[Grizzle]

What you suggested did work (a variation of anyway) but I found by putting the obj = getattr(obj, part) in a try/except block achieved the same thing and in theory should work for any other columns that have a similar issue.

    def render_column(self, row, column):
        """ Renders a column on a row. column can be given in a module notation eg. document.invoice.type
        """
        # try to find rightmost object
        obj = row
        parts = column.split('.')
        for part in parts[:-1]:
            if obj is None:
                break
            try:
                obj = getattr(obj, part)
            except:
                return '-'

        # try using get_OBJECT_display for choice fields
        if hasattr(obj, 'get_%s_display' % parts[-1]):
            value = getattr(obj, 'get_%s_display' % parts[-1])()
        else:
            value = getattr(obj, parts[-1], None)
        if value is None:
            value = self.none_string
        if self.escape_values:
            value = escape(value)
        if value and hasattr(obj, 'get_absolute_url'):
            return '<a href="%s">%s</a>' % (obj.get_absolute_url(), value)
        return value

I guess the other thing would be to return something appropriate to say that the column in empty.