# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.db.models.functions.base

    """
    Classes that represent database functions.
    """
    from django.db.models import Func, Transform, Value, fields

    class Cast(Func):
        """
        Coerce an expression to a new field type.
        """
        function = 'CAST'
        template = '%(function)s(%(expressions)s AS %(db_type)s)'

        mysql_types = {
            fields.CharField: 'char',
            fields.IntegerField: 'signed integer',
            fields.FloatField: 'signed',
        }

        def __init__(self, expression, output_field):
            super(Cast, self).__init__(expression, output_field=output_field)

        def as_sql(self, compiler, connection, **extra_context):
            if 'db_type' not in extra_context:
                extra_context['db_type'] = self._output_field.db_type(connection)
            return super(Cast, self).as_sql(compiler, connection, **extra_context)

        def as_mysql(self, compiler, connection):
            extra_context = {}
            output_field_class = type(self._output_field)
            if output_field_class in self.mysql_types:
                extra_context['db_type'] = self.mysql_types[output_field_class]
            return self.as_sql(compiler, connection, **extra_context)

        def as_postgresql(self, compiler, connection):
            # CAST would be valid too, but the :: shortcut syntax is more readable.
            return self.as_sql(compiler, connection, template='%(expressions)s::%(db_type)s')

    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()]
                clone = self.copy()
                clone.set_source_expressions(expressions)
                return super(Coalesce, clone).as_sql(compiler, connection)
            return 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()
            return super(ConcatPair, coalesced).as_sql(
                compiler, connection, template='%(expressions)s', arg_joiner=' || '
            )

        def as_mysql(self, compiler, connection):
            # Use CONCAT_WS with an empty separator so that NULLs are ignored.
            return super(ConcatPair, self).as_sql(
                compiler, connection, function='CONCAT_WS', template="%(function)s('', %(expressions)s)"
            )

        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 Greatest(Func):
        """
        Chooses the maximum expression and returns it.

        If any expression is null the return value is database-specific:
        On Postgres, 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(Greatest, self).__init__(*expressions, **extra)

        def as_sqlite(self, compiler, connection):
            """Use the MAX function on SQLite."""
            return super(Greatest, self).as_sql(compiler, connection, function='MAX')

    class Least(Func):
        """
        Chooses the minimum expression and returns it.

        If any expression is null the return value is database-specific:
        On Postgres, the minimum not-null expression is returned.
        On MySQL, Oracle, and SQLite, if any expression is null, null is returned.
        """
        function = 'LEAST'

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

        def as_sqlite(self, compiler, connection):
            """Use the MIN function on SQLite."""
            return super(Least, self).as_sql(compiler, connection, function='MIN')

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

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

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

    class Lower(Transform):
        function = 'LOWER'
        lookup_name = 'lower'

    class Now(Func):
        template = 'CURRENT_TIMESTAMP'

        def __init__(self, output_field=None, **extra):
            if output_field is None:
                output_field = fields.DateTimeField()
            super(Now, self).__init__(output_field=output_field, **extra)

        def as_postgresql(self, compiler, connection):
            # Postgres' CURRENT_TIMESTAMP means "the time at the start of the
            # transaction". We use STATEMENT_TIMESTAMP to be cross-compatible with
            # other databases.
            return self.as_sql(compiler, connection, template='STATEMENT_TIMESTAMP()')

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

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

    class Upper(Transform):
        function = 'UPPER'
        lookup_name = 'upper'