• Language: en
  • 2.1
  • Documentation version: development

Source code for django.db.models.functions.comparison

"""Database functions that do comparisons or type conversions."""
from django.db.models import Func


[docs]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_sqlite(self, compiler, connection, **extra_context): db_type = self.output_field.db_type(connection) if db_type in {'datetime', 'time'}: # Use strftime as datetime/time don't keep fractional seconds. template = 'strftime(%%s, %(expressions)s)' sql, params = super().as_sql(compiler, connection, template=template, **extra_context) format_string = '%H:%M:%f' if db_type == 'time' else '%Y-%m-%d %H:%M:%f' params.insert(0, format_string) return sql, params elif db_type == 'date': template = 'date(%(expressions)s)' return super().as_sql(compiler, connection, template=template, **extra_context) return self.as_sql(compiler, connection, **extra_context) def as_mysql(self, compiler, connection, **extra_context): # 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, **extra_context) def as_postgresql(self, compiler, connection, **extra_context): # 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', **extra_context)
[docs]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, **extra_context): # 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, **extra_context) return self.as_sql(compiler, connection, **extra_context)
[docs]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, **extra_context): """Use the MAX function on SQLite.""" return super().as_sqlite(compiler, connection, function='MAX', **extra_context)
[docs]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, **extra_context): """Use the MIN function on SQLite.""" return super().as_sqlite(compiler, connection, function='MIN', **extra_context)
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