Subclassing django's QuerySet

custom_manager.py

from django.db import models, connection


class SearchQuerySet(models.query.QuerySet):
    """SearchQuerySet"""
    def __init__(self, model=None, fields=None, using=None, query=None):
        super(SearchQuerySet, self).__init__(model, query, using)
        self._search_fields = fields
        
    def search(self, squery):
        """creates a sql match expresion like 
            SELECT field_name, MATCH(`product`.`name`, `product`.`description`)
                               AGAINST ('nike') AS 'relevance' FROM `product` WHERE
                               MATCH(`product`.`name`, `product`.`description`) AGAINST ('nike')
         """
        meta = self.model._meta
        columns = [meta.get_field(name) for name in self._search_fields]
        table_culumn_names = ["%s.%s" % (connection.ops.quote_name(meta.db_table), connection.ops.quote_name(field.column)) for field in columns]
        full_column_names = ", ".join(table_culumn_names)
        expr = ("MATCH(%s) AGAINST ('%s')" % (full_column_names, squery))
        return self.extra(select={'relevance': expr}, where=[expr])
            

class SearchManager(models.Manager):
    """SearchManager"""
    use_for_related_fields = True
    
    def __init__(self, fields):
        super(SearchManager, self).__init__()
        self._search_fields = fields
    
    def get_query_set(self):
        """overrides get_query_set method and loads the custom SearchQuerySet"""
        return SearchQuerySet(self.model, self._search_fields)
        
        
    def search(self, squery):
        """the custom search method applied to the model"""
        return self.get_query_set().search(squery)