"""
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, qn, connection):
"Return the aggregate, rendered as SQL."
if hasattr(self.col, 'as_sql'):
field_name = self.col.as_sql(qn, connection)
elif isinstance(self.col, (list, tuple)):
field_name = '.'.join([qn(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'
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