Transform a Variable compiler expression into its SQLAlchemy expression representation.

    Args:
        expression: expression, Variable compiler expression.
        node: SqlNode, the SqlNode the expression applies to.
        context: CompilationContext, global compilation state and metadata.

    Returns:
        Expression, SQLAlchemy expression.

def _transform_variable_to_expression(expression, node, context):
    """Transform a Variable compiler expression into its SQLAlchemy expression representation.

    Args:
        expression: expression, Variable compiler expression.
        node: SqlNode, the SqlNode the expression applies to.
        context: CompilationContext, global compilation state and metadata.

    Returns:
        Expression, SQLAlchemy expression.
    """
    variable_name = expression.variable_name
    if not variable_name.startswith(u'$'):
        raise AssertionError(u'Unexpectedly received variable name {} that is not '
                             u'prefixed with "$"'.format(variable_name))
    return bindparam(variable_name[1:])