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"""Database functions that do comparisons or type conversions."""
from django.db.models import Func
class Cast(Func):
"""Coerce an expression to a new field type."""
function = 'CAST'
template = '%(function)s(%(expressions)s AS %(db_type)s)'
def __init__(self, expression, output_field):
super().__init__(expression, output_field=output_field)
def as_sql(self, compiler, connection, **extra_context):
extra_context['db_type'] = self.output_field.cast_db_type(connection)
return super().as_sql(compiler, connection, **extra_context)
def as_mysql(self, compiler, connection):
# MySQL doesn't support explicit cast to float.
template = '(%(expressions)s + 0.0)' if self.output_field.get_internal_type() == 'FloatField' else None
return self.as_sql(compiler, connection, template=template)
def as_postgresql(self, compiler, connection):
# CAST would be valid too, but the :: shortcut syntax is more readable.
# 'expressions' is wrapped in parentheses in case it's a complex
# expression.
return self.as_sql(compiler, connection, template='(%(expressions)s)::%(db_type)s')
class Coalesce(Func):
"""Return, from left to right, the first non-null expression."""
function = 'COALESCE'
def __init__(self, *expressions, **extra):
if len(expressions) < 2:
raise ValueError('Coalesce must take at least two expressions')
super().__init__(*expressions, **extra)
def as_oracle(self, compiler, connection):
# Oracle prohibits mixing TextField (NCLOB) and CharField (NVARCHAR2),
# so convert all fields to NCLOB when that type is expected.
if self.output_field.get_internal_type() == 'TextField':
class ToNCLOB(Func):
function = 'TO_NCLOB'
expressions = [
ToNCLOB(expression) for expression in self.get_source_expressions()
]
clone = self.copy()
clone.set_source_expressions(expressions)
return super(Coalesce, clone).as_sql(compiler, connection)
return self.as_sql(compiler, connection)
class Greatest(Func):
"""
Return the maximum expression.
If any expression is null the return value is database-specific:
On PostgreSQL, the maximum not-null expression is returned.
On MySQL, Oracle, and SQLite, if any expression is null, null is returned.
"""
function = 'GREATEST'
def __init__(self, *expressions, **extra):
if len(expressions) < 2:
raise ValueError('Greatest must take at least two expressions')
super().__init__(*expressions, **extra)
def as_sqlite(self, compiler, connection):
"""Use the MAX function on SQLite."""
return super().as_sqlite(compiler, connection, function='MAX')
class Least(Func):
"""
Return the minimum expression.
If any expression is null the return value is database-specific:
On PostgreSQL, return the minimum not-null expression.
On MySQL, Oracle, and SQLite, if any expression is null, return null.
"""
function = 'LEAST'
def __init__(self, *expressions, **extra):
if len(expressions) < 2:
raise ValueError('Least must take at least two expressions')
super().__init__(*expressions, **extra)
def as_sqlite(self, compiler, connection):
"""Use the MIN function on SQLite."""
return super().as_sqlite(compiler, connection, function='MIN')
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