def _set_column_filter_expressions(self):
        """Construct the query: filtering.

        Add filtering when per column searching is used.
        """
        # per columns filters:
        for i in range(len(self.columns)):
            filter_expr = None
            value = self.params.get('columns[{:d}][search][value]'.format(i),
                                    '')
            if value:
                search_func = SEARCH_METHODS[self.columns[i].search_method]
                filter_expr = search_func(self.columns[i].sqla_expr, value)
            self.filter_expressions.append(filter_expr)
Construct the query: filtering.

        Add filtering when per column searching is used.