Provide a generic mechanism for building filter when missing is selected

[arthurzenika]

When building a facet with FacetedSearch one can specify to the TermsFacet the label for when the searched attribute is missing :

class MyFacetedSearch(FacetedSearch):
    facets = {
        'myattribute': TermsFacet(field='myattribute', missing='no myattribute')
}

The search generated by that item is the standard one and so will erroneously look for the "no myattribute" value in "myattribute".

Our hack so far to get this to work is the following :

class MyFacetedSearch(FacetedSearch):
    facets = {
        'myattribute': MyAttributeTermsFacet(field='myattribute', missing='no myattribute')
}
class MyAttributeTermsFacet(TermsFacet):
    def add_filter(self, filter_values):
        if 'no myattribute' in filter_values:
            return Q('missing', field='myattribute')
        return super(MyAttributeTermsFacet, self).add_filter(filter_values)

There must be a better way. Is there better way's place in the elasticsearch-dsl library ?