Fixed #3566 -- Added support for aggregation to the ORM. See the documentation for details on usage.

Many thanks to:
 * Nicolas Lara, who worked on this feature during the 2008 Google Summer of Code.
 * Alex Gaynor for his help debugging and fixing a number of issues.
 * Justin Bronn for his help integrating with contrib.gis.
 * Karen Tracey for her help with cross-platform testing.
 * Ian Kelly for his help testing and fixing Oracle support.
 * Malcolm Tredinnick for his invaluable review notes.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9742 bcc190cf-cafb-0310-a4f2-bffc1f526a37

4 files changed, 389 insertions, 131 deletions

diff --git a/django/db/models/sql/aggregates.py b/django/db/models/sql/aggregates.py
new file mode 100644
index 0000000000..6fdaf188c4
--- /dev/null
+++ b/django/db/models/sql/aggregates.py
@@ -0,0 +1,130 @@
+"""
+Classes to represent the default SQL aggregate functions
+"""
+
+class AggregateField(object):
+    """An internal field mockup used to identify aggregates in the
+    data-conversion parts of the database backend.
+    """
+    def __init__(self, internal_type):
+        self.internal_type = internal_type
+    def get_internal_type(self):
+        return self.internal_type
+
+ordinal_aggregate_field = AggregateField('IntegerField')
+computed_aggregate_field = AggregateField('FloatField')
+
+class Aggregate(object):
+    """
+    Default SQL Aggregate.
+    """
+    is_ordinal = False
+    is_computed = False
+    sql_template = '%(function)s(%(field)s)'
+
+    def __init__(self, col, source=None, is_summary=False, **extra):
+        """Instantiate an SQL aggregate
+
+         * col is a column reference describing the subject field
+           of the aggregate. It can be an alias, or a tuple describing
+           a table and column name.
+         * source is the underlying field or aggregate definition for
+           the column reference. If the aggregate is not an ordinal or
+           computed type, this reference is used to determine the coerced
+           output type of the aggregate.
+         * extra is a dictionary of additional data to provide for the
+           aggregate definition
+
+        Also utilizes the class variables:
+         * sql_function, the name of the SQL function that implements the
+           aggregate.
+         * sql_template, a template string that is used to render the
+           aggregate into SQL.
+         * is_ordinal, a boolean indicating if the output of this aggregate
+           is an integer (e.g., a count)
+         * is_computed, a boolean indicating if this output of this aggregate
+           is a computed float (e.g., an average), regardless of the input
+           type.
+
+        """
+        self.col = col
+        self.source = source
+        self.is_summary = is_summary
+        self.extra = extra
+
+        # Follow the chain of aggregate sources back until you find an
+        # actual field, or an aggregate that forces a particular output
+        # type. This type of this field will be used to coerce values
+        # retrieved from the database.
+        tmp = self
+
+        while tmp and isinstance(tmp, Aggregate):
+            if getattr(tmp, 'is_ordinal', False):
+                tmp = ordinal_aggregate_field
+            elif getattr(tmp, 'is_computed', False):
+                tmp = computed_aggregate_field
+            else:
+                tmp = tmp.source
+
+        self.field = tmp
+
+    def relabel_aliases(self, change_map):
+        if isinstance(self.col, (list, tuple)):
+            self.col = (change_map.get(self.col[0], self.col[0]), self.col[1])
+
+    def as_sql(self, quote_func=None):
+        "Return the aggregate, rendered as SQL."
+        if not quote_func:
+            quote_func = lambda x: x
+
+        if hasattr(self.col, 'as_sql'):
+            field_name = self.col.as_sql(quote_func)
+        elif isinstance(self.col, (list, tuple)):
+            field_name = '.'.join([quote_func(c) for c in self.col])
+        else:
+            field_name = self.col
+
+        params = {
+            'function': self.sql_function,
+            'field': field_name
+        }
+        params.update(self.extra)
+
+        return self.sql_template % params
+
+
+class Avg(Aggregate):
+    is_computed = True
+    sql_function = 'AVG'
+
+class Count(Aggregate):
+    is_ordinal = True
+    sql_function = 'COUNT'
+    sql_template = '%(function)s(%(distinct)s%(field)s)'
+
+    def __init__(self, col, distinct=False, **extra):
+        super(Count, self).__init__(col, distinct=distinct and 'DISTINCT ' or '', **extra)
+
+class Max(Aggregate):
+    sql_function = 'MAX'
+
+class Min(Aggregate):
+    sql_function = 'MIN'
+
+class StdDev(Aggregate):
+    is_computed = True
+
+    def __init__(self, col, sample=False, **extra):
+        super(StdDev, self).__init__(col, **extra)
+        self.sql_function = sample and 'STDDEV_SAMP' or 'STDDEV_POP'
+
+class Sum(Aggregate):
+    sql_function = 'SUM'
+
+class Variance(Aggregate):
+    is_computed = True
+
+    def __init__(self, col, sample=False, **extra):
+        super(Variance, self).__init__(col, **extra)
+        self.sql_function = sample and 'VAR_SAMP' or 'VAR_POP'
+
diff --git a/django/db/models/sql/datastructures.py b/django/db/models/sql/datastructures.py
index 913d8fde25..4d53999c79 100644
--- a/django/db/models/sql/datastructures.py
+++ b/django/db/models/sql/datastructures.py
@@ -25,59 +25,6 @@ class RawValue(object):
     def __init__(self, value):
         self.value = value
 
-class Aggregate(object):
-    """
-    Base class for all aggregate-related classes (min, max, avg, count, sum).
-    """
-    def relabel_aliases(self, change_map):
-        """
-        Relabel the column alias, if necessary. Must be implemented by
-        subclasses.
-        """
-        raise NotImplementedError
-
-    def as_sql(self, quote_func=None):
-        """
-        Returns the SQL string fragment for this object.
-
-        The quote_func function is used to quote the column components. If
-        None, it defaults to doing nothing.
-
-        Must be implemented by subclasses.
-        """
-        raise NotImplementedError
-
-class Count(Aggregate):
-    """
-    Perform a count on the given column.
-    """
-    def __init__(self, col='*', distinct=False):
-        """
-        Set the column to count on (defaults to '*') and set whether the count
-        should be distinct or not.
-        """
-        self.col = col
-        self.distinct = distinct
-
-    def relabel_aliases(self, change_map):
-        c = self.col
-        if isinstance(c, (list, tuple)):
-            self.col = (change_map.get(c[0], c[0]), c[1])
-
-    def as_sql(self, quote_func=None):
-        if not quote_func:
-            quote_func = lambda x: x
-        if isinstance(self.col, (list, tuple)):
-            col = ('%s.%s' % tuple([quote_func(c) for c in self.col]))
-        elif hasattr(self.col, 'as_sql'):
-            col = self.col.as_sql(quote_func)
-        else:
-            col = self.col
-        if self.distinct:
-            return 'COUNT(DISTINCT %s)' % col
-        else:
-            return 'COUNT(%s)' % col
-
 class Date(object):
     """
     Add a date selection column.
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index b12912461f..156617f807 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -12,12 +12,13 @@ from copy import deepcopy
 from django.utils.tree import Node
 from django.utils.datastructures import SortedDict
 from django.utils.encoding import force_unicode
+from django.db.backends.util import truncate_name
 from django.db import connection
 from django.db.models import signals
 from django.db.models.fields import FieldDoesNotExist
 from django.db.models.query_utils import select_related_descend
+from django.db.models.sql import aggregates as base_aggregates_module
 from django.db.models.sql.where import WhereNode, Constraint, EverythingNode, AND, OR
-from django.db.models.sql.datastructures import Count
 from django.core.exceptions import FieldError
 from datastructures import EmptyResultSet, Empty, MultiJoin
 from constants import *
@@ -40,6 +41,7 @@ class BaseQuery(object):
 
     alias_prefix = 'T'
     query_terms = QUERY_TERMS
+    aggregates_module = base_aggregates_module
 
     def __init__(self, model, connection, where=WhereNode):
         self.model = model
@@ -73,6 +75,9 @@ class BaseQuery(object):
         self.select_related = False
         self.related_select_cols = []
 
+        # SQL aggregate-related attributes
+        self.aggregate_select = SortedDict() # Maps alias -> SQL aggregate function
+
         # Arbitrary maximum limit for select_related. Prevents infinite
         # recursion. Can be changed by the depth parameter to select_related().
         self.max_depth = 5
@@ -178,6 +183,7 @@ class BaseQuery(object):
         obj.distinct = self.distinct
         obj.select_related = self.select_related
         obj.related_select_cols = []
+        obj.aggregate_select = self.aggregate_select.copy()
         obj.max_depth = self.max_depth
         obj.extra_select = self.extra_select.copy()
         obj.extra_tables = self.extra_tables
@@ -194,6 +200,35 @@ class BaseQuery(object):
             obj._setup_query()
         return obj
 
+    def convert_values(self, value, field):
+        """Convert the database-returned value into a type that is consistent
+        across database backends.
+
+        By default, this defers to the underlying backend operations, but
+        it can be overridden by Query classes for specific backends.
+        """
+        return self.connection.ops.convert_values(value, field)
+
+    def resolve_aggregate(self, value, aggregate):
+        """Resolve the value of aggregates returned by the database to
+        consistent (and reasonable) types.
+
+        This is required because of the predisposition of certain backends
+        to return Decimal and long types when they are not needed.
+        """
+        if value is None:
+            # Return None as-is
+            return value
+        elif aggregate.is_ordinal:
+            # Any ordinal aggregate (e.g., count) returns an int
+            return int(value)
+        elif aggregate.is_computed:
+            # Any computed aggregate (e.g., avg) returns a float
+            return float(value)
+        else:
+            # Return value depends on the type of the field being processed.
+            return self.convert_values(value, aggregate.field)
+
     def results_iter(self):
         """
         Returns an iterator over the results from executing this query.
@@ -212,29 +247,78 @@ class BaseQuery(object):
                         else:
                             fields = self.model._meta.fields
                     row = self.resolve_columns(row, fields)
+
+                if self.aggregate_select:
+                    aggregate_start = len(self.extra_select.keys()) + len(self.select)
+                    row = tuple(row[:aggregate_start]) + tuple([
+                        self.resolve_aggregate(value, aggregate)
+                        for (alias, aggregate), value
+                        in zip(self.aggregate_select.items(), row[aggregate_start:])
+                    ])
+
                 yield row
 
+    def get_aggregation(self):
+        """
+        Returns the dictionary with the values of the existing aggregations.
+        """
+        if not self.aggregate_select:
+            return {}
+
+        # If there is a group by clause, aggregating does not add useful
+        # information but retrieves only the first row. Aggregate
+        # over the subquery instead.
+        if self.group_by:
+            from subqueries import AggregateQuery
+            query = AggregateQuery(self.model, self.connection)
+
+            obj = self.clone()
+
+            # Remove any aggregates marked for reduction from the subquery
+            # and move them to the outer AggregateQuery.
+            for alias, aggregate in self.aggregate_select.items():
+                if aggregate.is_summary:
+                    query.aggregate_select[alias] = aggregate
+                    del obj.aggregate_select[alias]
+
+            query.add_subquery(obj)
+        else:
+            query = self
+            self.select = []
+            self.default_cols = False
+            self.extra_select = {}
+
+        query.clear_ordering(True)
+        query.clear_limits()
+        query.select_related = False
+        query.related_select_cols = []
+        query.related_select_fields = []
+
+        return dict([
+            (alias, self.resolve_aggregate(val, aggregate))
+            for (alias, aggregate), val
+            in zip(query.aggregate_select.items(), query.execute_sql(SINGLE))
+        ])
+
     def get_count(self):
         """
         Performs a COUNT() query using the current filter constraints.
         """
-        from subqueries import CountQuery
         obj = self.clone()
-        obj.clear_ordering(True)
-        obj.clear_limits()
-        obj.select_related = False
-        obj.related_select_cols = []
-        obj.related_select_fields = []
-        if len(obj.select) > 1:
-            obj = self.clone(CountQuery, _query=obj, where=self.where_class(),
-                    distinct=False)
-            obj.select = []
-            obj.extra_select = SortedDict()
+        if len(self.select) > 1:
+            # If a select clause exists, then the query has already started to
+            # specify the columns that are to be returned.
+            # In this case, we need to use a subquery to evaluate the count.
+            from subqueries import AggregateQuery
+            subquery = obj
+            subquery.clear_ordering(True)
+            subquery.clear_limits()
+
+            obj = AggregateQuery(obj.model, obj.connection)
+            obj.add_subquery(subquery)
+
         obj.add_count_column()
-        data = obj.execute_sql(SINGLE)
-        if not data:
-            return 0
-        number = data[0]
+        number = obj.get_aggregation()[None]
 
         # Apply offset and limit constraints manually, since using LIMIT/OFFSET
         # in SQL (in variants that provide them) doesn't change the COUNT
@@ -450,25 +534,41 @@ class BaseQuery(object):
             for col in self.select:
                 if isinstance(col, (list, tuple)):
                     r = '%s.%s' % (qn(col[0]), qn(col[1]))
-                    if with_aliases and col[1] in col_aliases:
-                        c_alias = 'Col%d' % len(col_aliases)
-                        result.append('%s AS %s' % (r, c_alias))
-                        aliases.add(c_alias)
-                        col_aliases.add(c_alias)
+                    if with_aliases:
+                        if col[1] in col_aliases:
+                            c_alias = 'Col%d' % len(col_aliases)
+                            result.append('%s AS %s' % (r, c_alias))
+                            aliases.add(c_alias)
+                            col_aliases.add(c_alias)
+                        else:
+                            result.append('%s AS %s' % (r, col[1]))
+                            aliases.add(r)
+                            col_aliases.add(col[1])
                     else:
                         result.append(r)
                         aliases.add(r)
                         col_aliases.add(col[1])
                 else:
                     result.append(col.as_sql(quote_func=qn))
+
                     if hasattr(col, 'alias'):
                         aliases.add(col.alias)
                         col_aliases.add(col.alias)
+
         elif self.default_cols:
             cols, new_aliases = self.get_default_columns(with_aliases,
                     col_aliases)
             result.extend(cols)
             aliases.update(new_aliases)
+
+        result.extend([
+            '%s%s' % (
+                aggregate.as_sql(quote_func=qn),
+                alias is not None and ' AS %s' % qn(alias) or ''
+            )
+            for alias, aggregate in self.aggregate_select.items()
+        ])
+
         for table, col in self.related_select_cols:
             r = '%s.%s' % (qn(table), qn(col))
             if with_aliases and col in col_aliases:
@@ -538,7 +638,7 @@ class BaseQuery(object):
         Returns a list of strings that are joined together to go after the
         "FROM" part of the query, as well as a list any extra parameters that
         need to be included. Sub-classes, can override this to create a
-        from-clause via a "select", for example (e.g. CountQuery).
+        from-clause via a "select".
 
         This should only be called after any SQL construction methods that
         might change the tables we need. This means the select columns and
@@ -635,10 +735,13 @@ class BaseQuery(object):
                     order = asc
                 result.append('%s %s' % (field, order))
                 continue
+            col, order = get_order_dir(field, asc)
+            if col in self.aggregate_select:
+                result.append('%s %s' % (col, order))
+                continue
             if '.' in field:
                 # This came in through an extra(order_by=...) addition. Pass it
                 # on verbatim.
-                col, order = get_order_dir(field, asc)
                 table, col = col.split('.', 1)
                 if (table, col) not in processed_pairs:
                     elt = '%s.%s' % (qn(table), col)
@@ -657,7 +760,6 @@ class BaseQuery(object):
                             ordering_aliases.append(elt)
                         result.append('%s %s' % (elt, order))
             else:
-                col, order = get_order_dir(field, asc)
                 elt = qn2(col)
                 if distinct and col not in select_aliases:
                     ordering_aliases.append(elt)
@@ -1068,6 +1170,48 @@ class BaseQuery(object):
             self.fill_related_selections(f.rel.to._meta, alias, cur_depth + 1,
                     used, next, restricted, new_nullable, dupe_set, avoid)
 
+    def add_aggregate(self, aggregate, model, alias, is_summary):
+        """
+        Adds a single aggregate expression to the Query
+        """
+        opts = model._meta
+        field_list = aggregate.lookup.split(LOOKUP_SEP)
+        if (len(field_list) == 1 and
+            aggregate.lookup in self.aggregate_select.keys()):
+            # Aggregate is over an annotation
+            field_name = field_list[0]
+            col = field_name
+            source = self.aggregate_select[field_name]
+        elif (len(field_list) > 1 or
+            field_list[0] not in [i.name for i in opts.fields]):
+            field, source, opts, join_list, last, _ = self.setup_joins(
+                field_list, opts, self.get_initial_alias(), False)
+
+            # Process the join chain to see if it can be trimmed
+            _, _, col, _, join_list = self.trim_joins(source, join_list, last, False)
+
+            # If the aggregate references a model or field that requires a join,
+            # those joins must be LEFT OUTER - empty join rows must be returned
+            # in order for zeros to be returned for those aggregates.
+            for column_alias in join_list:
+                self.promote_alias(column_alias, unconditional=True)
+
+            col = (join_list[-1], col)
+        else:
+            # Aggregate references a normal field
+            field_name = field_list[0]
+            source = opts.get_field(field_name)
+            if not (self.group_by and is_summary):
+                # Only use a column alias if this is a
+                # standalone aggregate, or an annotation
+                col = (opts.db_table, source.column)
+            else:
+                col = field_name
+
+        # Add the aggregate to the query
+        alias = truncate_name(alias, self.connection.ops.max_name_length())
+        aggregate.add_to_query(self, alias, col=col, source=source, is_summary=is_summary)
+
     def add_filter(self, filter_expr, connector=AND, negate=False, trim=False,
             can_reuse=None, process_extras=True):
         """
@@ -1119,6 +1263,11 @@ class BaseQuery(object):
         elif callable(value):
             value = value()
 
+        for alias, aggregate in self.aggregate_select.items():
+            if alias == parts[0]:
+                self.having.add((aggregate, lookup_type, value), AND)
+                return
+
         opts = self.get_meta()
         alias = self.get_initial_alias()
         allow_many = trim or not negate
@@ -1131,38 +1280,9 @@ class BaseQuery(object):
             self.split_exclude(filter_expr, LOOKUP_SEP.join(parts[:e.level]),
                     can_reuse)
             return
-        final = len(join_list)
-        penultimate = last.pop()
-        if penultimate == final:
-            penultimate = last.pop()
-        if trim and len(join_list) > 1:
-            extra = join_list[penultimate:]
-            join_list = join_list[:penultimate]
-            final = penultimate
-            penultimate = last.pop()
-            col = self.alias_map[extra[0]][LHS_JOIN_COL]
-            for alias in extra:
-                self.unref_alias(alias)
-        else:
-            col = target.column
-        alias = join_list[-1]
 
-        while final > 1:
-            # An optimization: if the final join is against the same column as
-            # we are comparing against, we can go back one step in the join
-            # chain and compare against the lhs of the join instead (and then
-            # repeat the optimization). The result, potentially, involves less
-            # table joins.
-            join = self.alias_map[alias]
-            if col != join[RHS_JOIN_COL]:
-                break
-            self.unref_alias(alias)
-            alias = join[LHS_ALIAS]
-            col = join[LHS_JOIN_COL]
-            join_list = join_list[:-1]
-            final -= 1
-            if final == penultimate:
-                penultimate = last.pop()
+        # Process the join chain to see if it can be trimmed
+        final, penultimate, col, alias, join_list = self.trim_joins(target, join_list, last, trim)
 
         if (lookup_type == 'isnull' and value is True and not negate and
                 final > 1):
@@ -1313,7 +1433,7 @@ class BaseQuery(object):
                         field, model, direct, m2m = opts.get_field_by_name(f.name)
                         break
                 else:
-                    names = opts.get_all_field_names()
+                    names = opts.get_all_field_names() + self.aggregate_select.keys()
                     raise FieldError("Cannot resolve keyword %r into field. "
                             "Choices are: %s" % (name, ", ".join(names)))
 
@@ -1462,6 +1582,43 @@ class BaseQuery(object):
 
         return field, target, opts, joins, last, extra_filters
 
+    def trim_joins(self, target, join_list, last, trim):
+        """An optimization: if the final join is against the same column as
+        we are comparing against, we can go back one step in a join
+        chain and compare against the LHS of the join instead (and then
+        repeat the optimization). The result, potentially, involves less
+        table joins.
+
+        Returns a tuple
+        """
+        final = len(join_list)
+        penultimate = last.pop()
+        if penultimate == final:
+            penultimate = last.pop()
+        if trim and len(join_list) > 1:
+            extra = join_list[penultimate:]
+            join_list = join_list[:penultimate]
+            final = penultimate
+            penultimate = last.pop()
+            col = self.alias_map[extra[0]][LHS_JOIN_COL]
+            for alias in extra:
+                self.unref_alias(alias)
+        else:
+            col = target.column
+        alias = join_list[-1]
+        while final > 1:
+            join = self.alias_map[alias]
+            if col != join[RHS_JOIN_COL]:
+                break
+            self.unref_alias(alias)
+            alias = join[LHS_ALIAS]
+            col = join[LHS_JOIN_COL]
+            join_list = join_list[:-1]
+            final -= 1
+            if final == penultimate:
+                penultimate = last.pop()
+        return final, penultimate, col, alias, join_list
+
     def update_dupe_avoidance(self, opts, col, alias):
         """
         For a column that is one of multiple pointing to the same table, update
@@ -1554,6 +1711,7 @@ class BaseQuery(object):
         """
         alias = self.get_initial_alias()
         opts = self.get_meta()
+
         try:
             for name in field_names:
                 field, target, u2, joins, u3, u4 = self.setup_joins(
@@ -1574,7 +1732,7 @@ class BaseQuery(object):
         except MultiJoin:
             raise FieldError("Invalid field name: '%s'" % name)
         except FieldError:
-            names = opts.get_all_field_names() + self.extra_select.keys()
+            names = opts.get_all_field_names() + self.extra_select.keys() + self.aggregate_select.keys()
             names.sort()
             raise FieldError("Cannot resolve keyword %r into field. "
                     "Choices are: %s" % (name, ", ".join(names)))
@@ -1609,38 +1767,52 @@ class BaseQuery(object):
         if force_empty:
             self.default_ordering = False
 
+    def set_group_by(self):
+        """
+        Expands the GROUP BY clause required by the query.
+
+        This will usually be the set of all non-aggregate fields in the
+        return data. If the database backend supports grouping by the
+        primary key, and the query would be equivalent, the optimization
+        will be made automatically.
+        """
+        if self.connection.features.allows_group_by_pk:
+            if len(self.select) == len(self.model._meta.fields):
+                self.group_by.append('.'.join([self.model._meta.db_table,
+                                               self.model._meta.pk.column]))
+                return
+
+        for sel in self.select:
+            self.group_by.append(sel)
+
     def add_count_column(self):
         """
         Converts the query to do count(...) or count(distinct(pk)) in order to
         get its size.
         """
-        # TODO: When group_by support is added, this needs to be adjusted so
-        # that it doesn't totally overwrite the select list.
         if not self.distinct:
             if not self.select:
-                select = Count()
+                count = self.aggregates_module.Count('*', is_summary=True)
             else:
                 assert len(self.select) == 1, \
                         "Cannot add count col with multiple cols in 'select': %r" % self.select
-                select = Count(self.select[0])
+                count = self.aggregates_module.Count(self.select[0])
         else:
             opts = self.model._meta
             if not self.select:
-                select = Count((self.join((None, opts.db_table, None, None)),
-                        opts.pk.column), True)
+                count = self.aggregates_module.Count((self.join((None, opts.db_table, None, None)), opts.pk.column),
+                                         is_summary=True, distinct=True)
             else:
                 # Because of SQL portability issues, multi-column, distinct
                 # counts need a sub-query -- see get_count() for details.
                 assert len(self.select) == 1, \
                         "Cannot add count col with multiple cols in 'select'."
-                select = Count(self.select[0], True)
 
+                count = self.aggregates_module.Count(self.select[0], distinct=True)
             # Distinct handling is done in Count(), so don't do it at this
             # level.
             self.distinct = False
-        self.select = [select]
-        self.select_fields = [None]
-        self.extra_select = {}
+        self.aggregate_select = {None: count}
 
     def add_select_related(self, fields):
         """
@@ -1758,7 +1930,6 @@ class BaseQuery(object):
                 return empty_iter()
             else:
                 return
-
         cursor = self.connection.cursor()
         cursor.execute(sql, params)
 
diff --git a/django/db/models/sql/subqueries.py b/django/db/models/sql/subqueries.py
index 524b0894c5..0a59b403c8 100644
--- a/django/db/models/sql/subqueries.py
+++ b/django/db/models/sql/subqueries.py
@@ -9,7 +9,7 @@ from django.db.models.sql.query import Query
 from django.db.models.sql.where import AND, Constraint
 
 __all__ = ['DeleteQuery', 'UpdateQuery', 'InsertQuery', 'DateQuery',
-        'CountQuery']
+        'AggregateQuery']
 
 class DeleteQuery(Query):
     """
@@ -400,15 +400,25 @@ class DateQuery(Query):
         self.distinct = True
         self.order_by = order == 'ASC' and [1] or [-1]
 
-class CountQuery(Query):
+class AggregateQuery(Query):
     """
-    A CountQuery knows how to take a normal query which would select over
-    multiple distinct columns and turn it into SQL that can be used on a
-    variety of backends (it requires a select in the FROM clause).
+    An AggregateQuery takes another query as a parameter to the FROM
+    clause and only selects the elements in the provided list.
     """
-    def get_from_clause(self):
-        result, params = self._query.as_sql()
-        return ['(%s) A1' % result], params
+    def add_subquery(self, query):
+        self.subquery, self.sub_params = query.as_sql(with_col_aliases=True)
 
-    def get_ordering(self):
-        return ()
+    def as_sql(self, quote_func=None):
+        """
+        Creates the SQL for this query. Returns the SQL string and list of
+        parameters.
+        """
+        sql = ('SELECT %s FROM (%s) subquery' % (
+            ', '.join([
+                aggregate.as_sql()
+                for aggregate in self.aggregate_select.values()
+            ]),
+            self.subquery)
+        )
+        params = self.sub_params
+        return (sql, params)