from django.conf import settings
from django.db import models
from django.core.files.storage import get_storage_class, FileSystemStorage
from django.core.exceptions import ValidationError
import json

STORAGE_CLASSES = getattr(settings, 'STORAGE_CLASSES', [])

STORAGE_CLASSES.extend([
    'django.core.files.storage.FileSystemStorage',
])

try:
    import storages
    STORAGE_CLASSES.append('storages.backends.s3boto3.S3Boto3Storage')
except ImportError:
    pass

STORAGE_CLASS_OPTIONS = [ (x, x) for x in STORAGE_CLASSES ]

def validate_json(value):
    try:
        json.loads(value)
    except json.JSONDecodeError as e:
        raise ValidationError(e.msg)

class UserStorage(models.Model):
    """ A user defined storage
    """
    owner = models.ForeignKey('auth.User',
            on_delete=models.CASCADE,
            null=True, blank=True)
    name = models.SlugField(max_length=20,
            unique=True,
            help_text="Storage tag")
    storage = models.CharField(max_length=255,
            choices=STORAGE_CLASS_OPTIONS,
            help_text="Storage class for this instance")
    settings_data = models.TextField(
            validators=[validate_json],
            help_text="JSON dict with key/value settings")

    def instance(self):
        return get_storage_class(self.storage)(**self.settings)

    @property
    def settings(self):
        if not self.settings_data:
            return {}
        try:
            return json.loads(self.settings_data)
        except Exception as e:
            raise ValueError("Error in settings for storage '{0}' [{1}]".format(self.name, e))

    def clean(self):
        try:
            self.test_storage()
        except Exception as e:
            raise ValidationError(str(e))

    def test_storage(self):
        # just do something that requires connection
        try:
            self.instance().listdir('')
        except FileNotFoundError:
            # FileSystemStorage doesn't create the base_dir until write
            pass
        return True

    def save(self, *args, **kwargs):
        # ensure the settings JSON isvalid
        if self.settings_data:
            validate_json(self.settings_data)

        super(UserStorage, self).save(*args, **kwargs)

    def __str__(self):
        return self.name