# Django 1.8.18.dev20170123152445 documentation

# Source code for django.db.models.functions

    """
    Classes that represent database functions.
    """
    from django.db.models import IntegerField
    from django.db.models.expressions import Func, Value

    class Coalesce(Func):
        """
        Chooses, from left to right, the first non-null expression and returns it.
        """
        function = 'COALESCE'

        def __init__(self, *expressions, **extra):
            if len(expressions) < 2:
                raise ValueError('Coalesce must take at least two expressions')
            super(Coalesce, self).__init__(*expressions, **extra)

        def as_oracle(self, compiler, connection):
            # we can't mix TextField (NCLOB) and CharField (NVARCHAR), so convert
            # all fields to NCLOB when we expect NCLOB
            if self.output_field.get_internal_type() == 'TextField':
                class ToNCLOB(Func):
                    function = 'TO_NCLOB'

                expressions = [
                    ToNCLOB(expression) for expression in self.get_source_expressions()]
                self.set_source_expressions(expressions)
            return super(Coalesce, self).as_sql(compiler, connection)

    class ConcatPair(Func):
        """
        A helper class that concatenates two arguments together. This is used
        by `Concat` because not all backend databases support more than two
        arguments.
        """
        function = 'CONCAT'

        def __init__(self, left, right, **extra):
            super(ConcatPair, self).__init__(left, right, **extra)

        def as_sqlite(self, compiler, connection):
            coalesced = self.coalesce()
            coalesced.arg_joiner = ' || '
            coalesced.template = '%(expressions)s'
            return super(ConcatPair, coalesced).as_sql(compiler, connection)

        def as_mysql(self, compiler, connection):
            coalesced = self.coalesce()
            return super(ConcatPair, coalesced).as_sql(compiler, connection)

        def coalesce(self):
            # null on either side results in null for expression, wrap with coalesce
            c = self.copy()
            expressions = [
                Coalesce(expression, Value('')) for expression in c.get_source_expressions()
            ]
            c.set_source_expressions(expressions)
            return c

    class Concat(Func):
        """
        Concatenates text fields together. Backends that result in an entire
        null expression when any arguments are null will wrap each argument in
        coalesce functions to ensure we always get a non-null result.
        """
        function = None
        template = "%(expressions)s"

        def __init__(self, *expressions, **extra):
            if len(expressions) < 2:
                raise ValueError('Concat must take at least two expressions')
            paired = self._paired(expressions)
            super(Concat, self).__init__(paired, **extra)

        def _paired(self, expressions):
            # wrap pairs of expressions in successive concat functions
            # exp = [a, b, c, d]
            # -> ConcatPair(a, ConcatPair(b, ConcatPair(c, d))))
            if len(expressions) == 2:
                return ConcatPair(*expressions)
            return ConcatPair(expressions[0], self._paired(expressions[1:]))

    class Length(Func):
        """Returns the number of characters in the expression"""
        function = 'LENGTH'

        def __init__(self, expression, **extra):
            output_field = extra.pop('output_field', IntegerField())
            super(Length, self).__init__(expression, output_field=output_field, **extra)

        def as_mysql(self, compiler, connection):
            self.function = 'CHAR_LENGTH'
            return super(Length, self).as_sql(compiler, connection)

    class Lower(Func):
        function = 'LOWER'

        def __init__(self, expression, **extra):
            super(Lower, self).__init__(expression, **extra)

    class Substr(Func):
        function = 'SUBSTRING'

        def __init__(self, expression, pos, length=None, **extra):
            """
            expression: the name of a field, or an expression returning a string
            pos: an integer > 0, or an expression returning an integer
            length: an optional number of characters to return
            """
            if not hasattr(pos, 'resolve_expression'):
                if pos < 1:
                    raise ValueError("'pos' must be greater than 0")
                pos = Value(pos)
            expressions = [expression, pos]
            if length is not None:
                if not hasattr(length, 'resolve_expression'):
                    length = Value(length)
                expressions.append(length)
            super(Substr, self).__init__(*expressions, **extra)

        def as_sqlite(self, compiler, connection):
            self.function = 'SUBSTR'
            return super(Substr, self).as_sql(compiler, connection)

        def as_oracle(self, compiler, connection):
            self.function = 'SUBSTR'
            return super(Substr, self).as_sql(compiler, connection)

    class Upper(Func):
        function = 'UPPER'

        def __init__(self, expression, **extra):
            super(Upper, self).__init__(expression, **extra)