Group Concat aggregate for Django 1.4

django_concat.py

from django.db.models import Aggregate, fields
from django.db.models.sql.aggregates import Aggregate as SQLAggregate

class SQLConcat(SQLAggregate):
    sql_function = 'GROUP_CONCAT'
    sql_template = ('%(function)s(%(distinct)s%(field)s'
                    ' SEPARATOR "%(separator)s")')
    def __init__(self, col, separator=', ', distinct=False, **extra):
        super(SQLConcat, self).__init__(col, separator=separator,
                distinct=distinct and 'DISTINCT ' or '', **extra)

class Concat(Aggregate):
    def add_to_query(self, query, alias, col, source, is_summary):
        # This is a hack for 1.4 compatibility.
        # If your version is > 1.4, try without.
        source = fields.DateTimeField()
        # End hack
        aggregate = SQLConcat(col, source=source, is_summary=is_summary,
                **self.extra)
        query.aggregates[alias] = aggregate
    name = 'Concat'

## Example
# (Book.objects.select_related('author')
#    .values('author__name')
#    .annotate(titles=Concat('title', distinct=True, separator='; '))
# )