1 """

     2 Classes to represent the definitions of aggregate functions.

     3 """

     4 

     5 class Aggregate(object):

     6     """

     7     Default Aggregate definition.

     8     """

     9     def __init__(self, lookup, **extra):

    10         """Instantiate a new aggregate.

    11 

    12          * lookup is the field on which the aggregate operates.

    13          * extra is a dictionary of additional data to provide for the

    14            aggregate definition

    15 

    16         Also utilizes the class variables:

    17          * name, the identifier for this aggregate function.

    18         """

    19         self.lookup = lookup

    20         self.extra = extra

    21 

    22     def _default_alias(self):

    23         return '%s__%s' % (self.lookup, self.name.lower())

    24     default_alias = property(_default_alias)

    25 

    26     def add_to_query(self, query, alias, col, source, is_summary):

    27         """Add the aggregate to the nominated query.

    28 

    29         This method is used to convert the generic Aggregate definition into a

    30         backend-specific definition.

    31 

    32          * query is the backend-specific query instance to which the aggregate

    33            is to be added.

    34          * col is a column reference describing the subject field

    35            of the aggregate. It can be an alias, or a tuple describing

    36            a table and column name.

    37          * source is the underlying field or aggregate definition for

    38            the column reference. If the aggregate is not an ordinal or

    39            computed type, this reference is used to determine the coerced

    40            output type of the aggregate.

    41          * is_summary is a boolean that is set True if the aggregate is a

    42            summary value rather than an annotation.

    43         """

    44         klass = getattr(query.aggregates_module, self.name)

    45         aggregate = klass(col, source=source, is_summary=is_summary, **self.extra)

    46         query.aggregates[alias] = aggregate

    47 

    48 class Avg(Aggregate):

    49     name = 'Avg'

    50 

    51 class Count(Aggregate):

    52     name = 'Count'

    53 

    54 class Max(Aggregate):

    55     name = 'Max'

    56 

    57 class Min(Aggregate):

    58     name = 'Min'

    59 

    60 class StdDev(Aggregate):

    61     name = 'StdDev'

    62 

    63 class Sum(Aggregate):

    64     name = 'Sum'

    65 

    66 class Variance(Aggregate):

    67     name = 'Variance'