import inspect

from django.db import models
from django.db.models.manager import Manager


# from graphene.utils import LazyList


class LazyList(object):
    pass


try:
    import django_filters  # noqa
    DJANGO_FILTER_INSTALLED = True
except ImportError:
    DJANGO_FILTER_INSTALLED = False


def get_reverse_fields(model, local_field_names):
    for name, attr in model.__dict__.items():
        # Don't duplicate any local fields
        if name in local_field_names:
            continue

        # Django =>1.9 uses 'rel', django <1.9 uses 'related'
        related = getattr(attr, 'rel', None) or \
            getattr(attr, 'related', None)
        if isinstance(related, models.ManyToOneRel):
            yield (name, related)
        elif isinstance(related, models.ManyToManyRel) and not related.symmetrical:
            yield (name, related)


def maybe_queryset(value):
    if isinstance(value, Manager):
        value = value.get_queryset()
    return value


def get_model_fields(model):
    local_fields = [
        (field.name, field)
        for field
        in sorted(list(model._meta.fields) +
                  list(model._meta.local_many_to_many))
    ]

    # Make sure we don't duplicate local fields with "reverse" version
    local_field_names = [field[0] for field in local_fields]
    reverse_fields = get_reverse_fields(model, local_field_names)

    all_fields = local_fields + list(reverse_fields)

    return all_fields


def is_valid_django_model(model):
    return inspect.isclass(model) and issubclass(model, models.Model)


def import_single_dispatch():
    try:
        from functools import singledispatch
    except ImportError:
        singledispatch = None

    if not singledispatch:
        try:
            from singledispatch import singledispatch
        except ImportError:
            pass

    if not singledispatch:
        raise Exception(
            "It seems your python version does not include "
            "functools.singledispatch. Please install the 'singledispatch' "
            "package. More information here: "
            "https://pypi.python.org/pypi/singledispatch"
        )

    return singledispatch