126 lines
4.1 KiB
Python
126 lines
4.1 KiB
Python
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"""
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Classes that represent database functions.
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"""
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from django.db.models import IntegerField
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from django.db.models.expressions import Func, Value
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class Coalesce(Func):
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"""
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Chooses, from left to right, the first non-null expression and returns it.
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"""
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function = 'COALESCE'
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def __init__(self, *expressions, **extra):
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if len(expressions) < 2:
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raise ValueError('Coalesce must take at least two expressions')
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super(Coalesce, self).__init__(*expressions, **extra)
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class ConcatPair(Func):
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"""
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A helper class that concatenates two arguments together. This is used
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by `Concat` because not all backend databases support more than two
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arguments.
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"""
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function = 'CONCAT'
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def __init__(self, left, right, **extra):
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super(ConcatPair, self).__init__(left, right, **extra)
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def as_sqlite(self, compiler, connection):
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self.arg_joiner = ' || '
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self.template = '%(expressions)s'
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self.coalesce()
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return super(ConcatPair, self).as_sql(compiler, connection)
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def as_mysql(self, compiler, connection):
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self.coalesce()
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return super(ConcatPair, self).as_sql(compiler, connection)
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def coalesce(self):
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# null on either side results in null for expression, wrap with coalesce
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expressions = [
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Coalesce(expression, Value('')) for expression in self.get_source_expressions()]
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self.set_source_expressions(expressions)
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class Concat(Func):
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"""
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Concatenates text fields together. Backends that result in an entire
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null expression when any arguments are null will wrap each argument in
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coalesce functions to ensure we always get a non-null result.
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"""
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function = None
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template = "%(expressions)s"
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def __init__(self, *expressions, **extra):
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if len(expressions) < 2:
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raise ValueError('Concat must take at least two expressions')
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paired = self._paired(expressions)
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super(Concat, self).__init__(paired, **extra)
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def _paired(self, expressions):
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# wrap pairs of expressions in successive concat functions
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# exp = [a, b, c, d]
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# -> ConcatPair(a, ConcatPair(b, ConcatPair(c, d))))
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if len(expressions) == 2:
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return ConcatPair(*expressions)
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return ConcatPair(expressions[0], self._paired(expressions[1:]))
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class Length(Func):
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"""Returns the number of characters in the expression"""
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function = 'LENGTH'
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def __init__(self, expression, **extra):
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output_field = extra.pop('output_field', IntegerField())
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super(Length, self).__init__(expression, output_field=output_field, **extra)
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def as_mysql(self, compiler, connection):
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self.function = 'CHAR_LENGTH'
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return super(Length, self).as_sql(compiler, connection)
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class Lower(Func):
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function = 'LOWER'
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def __init__(self, expression, **extra):
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super(Lower, self).__init__(expression, **extra)
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class Substr(Func):
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function = 'SUBSTRING'
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def __init__(self, expression, pos, length=None, **extra):
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"""
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expression: the name of a field, or an expression returning a string
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pos: an integer > 0, or an expression returning an integer
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length: an optional number of characters to return
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"""
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if not hasattr('pos', 'resolve_expression'):
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if pos < 1:
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raise ValueError("'pos' must be greater than 0")
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pos = Value(pos)
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expressions = [expression, pos]
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if length is not None:
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if not hasattr('length', 'resolve_expression'):
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length = Value(length)
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expressions.append(length)
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super(Substr, self).__init__(*expressions, **extra)
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def as_sqlite(self, compiler, connection):
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self.function = 'SUBSTR'
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return super(Substr, self).as_sql(compiler, connection)
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def as_oracle(self, compiler, connection):
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self.function = 'SUBSTR'
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return super(Substr, self).as_sql(compiler, connection)
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class Upper(Func):
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function = 'UPPER'
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def __init__(self, expression, **extra):
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super(Upper, self).__init__(expression, **extra)
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