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

1 files changed, 66 insertions, 0 deletions

diff --git a/django/db/models/aggregates.py b/django/db/models/aggregates.py
new file mode 100644
index 0000000000..1f676253b5
--- /dev/null
+++ b/django/db/models/aggregates.py
@@ -0,0 +1,66 @@
+"""
+Classes to represent the definitions of aggregate functions.
+"""
+
+class Aggregate(object):
+    """
+    Default Aggregate definition.
+    """
+    def __init__(self, lookup, **extra):
+        """Instantiate a new aggregate.
+
+         * lookup is the field on which the aggregate operates.
+         * extra is a dictionary of additional data to provide for the
+           aggregate definition
+
+        Also utilizes the class variables:
+         * name, the identifier for this aggregate function.
+        """
+        self.lookup = lookup
+        self.extra = extra
+
+    def _default_alias(self):
+        return '%s__%s' % (self.lookup, self.name.lower())
+    default_alias = property(_default_alias)
+
+    def add_to_query(self, query, alias, col, source, is_summary):
+        """Add the aggregate to the nominated query.
+
+        This method is used to convert the generic Aggregate definition into a
+        backend-specific definition.
+
+         * query is the backend-specific query instance to which the aggregate
+           is to be added.
+         * 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.
+         * is_summary is a boolean that is set True if the aggregate is a
+           summary value rather than an annotation.
+        """
+        aggregate = getattr(query.aggregates_module, self.name)
+        query.aggregate_select[alias] = aggregate(col, source=source, is_summary=is_summary, **self.extra)
+
+class Avg(Aggregate):
+    name = 'Avg'
+
+class Count(Aggregate):
+    name = 'Count'
+
+class Max(Aggregate):
+    name = 'Max'
+
+class Min(Aggregate):
+    name = 'Min'
+
+class StdDev(Aggregate):
+    name = 'StdDev'
+
+class Sum(Aggregate):
+    name = 'Sum'
+
+class Variance(Aggregate):
+    name = 'Variance'