Django making a list of a field grouping by another field in model

Question

I have a model called MyModel which has some dummy data as follows:

     item    date     value
------------------------------
      ab    8/10/12   124
      ab    7/10/12   433
      ab    6/10/12    99
      abc   8/10/12    23
      abc   7/10/12    80

I would like to query this Model in such a way that i get an output as follows:

[{'item': 'ab', 'values': [ 124, 433, 99]},
 {'item': 'abc', 'values': [ 23, 80]}]

How would i be able to do this using the django ORM?

Answer 1

(Apr 4 '16) UPDATE: This is a working solution for Django <= 1.7. For newer versions please read Creating your own Aggregate Functions from the docs.

Using a custom Concat aggregate taken from here (an article about the topic)

Define this:

class Concat(models.Aggregate):
    def add_to_query(self, query, alias, col, source, is_summary):
        #we send source=CharField to prevent Django from casting string to int
        aggregate = SQLConcat(col, source=models.CharField(), is_summary=is_summary, **self.extra)
        query.aggregates[alias] = aggregate

#for mysql
class SQLConcat(models.sql.aggregates.Aggregate):
    sql_function = 'group_concat'

    @property
    def sql_template(self):
        if self.extra.get('separator'):
            return '%(function)s(%(field)s SEPARATOR "%(separator)s")'
        else:
            return '%(function)s(%(field)s)'

#For PostgreSQL >= 9.0
#Aways use with separator, e.g. .annotate(values=Concat('value', separator=','))     
class SQLConcat(models.sql.aggregates.Aggregate):
    sql_function = 'string_agg'

    @property
    def sql_template(self):
        #the ::text cast is a hardcoded hack to work with integer columns
        return "%(function)s(%(field)s::text, '%(separator)s')"

#For PostgreSQL >= 8.4 and < 9.0
#Aways use with separator, e.g. .annotate(values=Concat('value', separator=','))     
class SQLConcat(models.sql.aggregates.Aggregate):
    sql_function = 'array_to_string'

    @property
    def sql_template(self):
        return "%(function)s(array_agg(%(field)s), '%(separator)s')"

#For PostgreSQL < 8.4 you should define array_agg before using it:
#CREATE AGGREGATE array_agg (anyelement)
#(
#    sfunc = array_append,
#    stype = anyarray,
#    initcond = '{}'
#);

class MyModel(models.Model):
    item = models.CharField(max_length = 255)
    date = models.DateTimeField()
    value = models.IntegerField()

so now you can do:

>>> from my_app.models import MyModel, Concat
>>> MyModel.objects.values('item').annotate(values=Concat('value'))
[{'item': u'ab', 'values': u'124,433,99'}, {'item': u'abc', 'values': u'23,80'}]

to get values as a list of integers you need to manually .split and cast to int. Something like:

>>> my_list = MyModel.objects.values('item').annotate(values=Concat('value'))
>>> for i in my_list:
...     i['values'] = [int(v) for v in i['values'].split(',')]
...
>>> my_list
[{'item': u'ab', 'values': [124, 433, 99]}, {'item': u'abc', 'values': [23, 80]}]