import inspect
import types
from collections import defaultdict
from itertools import chain

from django.apps import apps
from django.conf import settings
from django.core.checks import Error, Tags, Warning, register


@register(Tags.models)
def check_all_models(app_configs=None, **kwargs):
    db_table_models = defaultdict(list)
    errors = []
    if app_configs is None:
        models = apps.get_models()
    else:
        models = chain.from_iterable(app_config.get_models() for app_config in app_configs)
    for model in models:
        if model._meta.managed and not model._meta.proxy:
            db_table_models[model._meta.db_table].append(model._meta.label)
        if not inspect.ismethod(model.check):
            errors.append(
                Error(
                    "The '%s.check()' class method is currently overridden by %r."
                    % (model.__name__, model.check),
                    obj=model,
                    id='models.E020'
                )
            )
        else:
            errors.extend(model.check(**kwargs))
    if settings.DATABASE_ROUTERS:
        error_class, error_id = Warning, 'models.W035'
        error_hint = (
            'You have configured settings.DATABASE_ROUTERS. Verify that %s '
            'are correctly routed to separate databases.'
        )
    else:
        error_class, error_id = Error, 'models.E028'
        error_hint = None
    for db_table, model_labels in db_table_models.items():
        if len(model_labels) != 1:
            model_labels_str = ', '.join(model_labels)
            errors.append(
                error_class(
                    "db_table '%s' is used by multiple models: %s."
                    % (db_table, model_labels_str),
                    obj=db_table,
                    hint=(error_hint % model_labels_str) if error_hint else None,
                    id=error_id,
                )
            )
    return errors


def _check_lazy_references(apps, ignore=None):
    """
    Ensure all lazy (i.e. string) model references have been resolved.

    Lazy references are used in various places throughout Django, primarily in
    related fields and model signals. Identify those common cases and provide
    more helpful error messages for them.

    The ignore parameter is used by StateApps to exclude swappable models from
    this check.
    """
    pending_models = set(apps._pending_operations) - (ignore or set())

    # Short circuit if there aren't any errors.
    if not pending_models:
        return []

    from django.db.models import signals
    model_signals = {
        signal: name for name, signal in vars(signals).items()
        if isinstance(signal, signals.ModelSignal)
    }

    def extract_operation(obj):
        """
        Take a callable found in Apps._pending_operations and identify the
        original callable passed to Apps.lazy_model_operation(). If that
        callable was a partial, return the inner, non-partial function and
        any arguments and keyword arguments that were supplied with it.

        obj is a callback defined locally in Apps.lazy_model_operation() and
        annotated there with a `func` attribute so as to imitate a partial.
        """
        operation, args, keywords = obj, [], {}
        while hasattr(operation, 'func'):
            # The or clauses are redundant but work around a bug (#25945) in
            # functools.partial in Python <= 3.5.1.
            args.extend(getattr(operation, 'args', []) or [])
            keywords.update(getattr(operation, 'keywords', {}) or {})
            operation = operation.func
        return operation, args, keywords

    def app_model_error(model_key):
        try:
            apps.get_app_config(model_key[0])
            model_error = "app '%s' doesn't provide model '%s'" % model_key
        except LookupError:
            model_error = "app '%s' isn't installed" % model_key[0]
        return model_error

    # Here are several functions which return CheckMessage instances for the
    # most common usages of lazy operations throughout Django. These functions
    # take the model that was being waited on as an (app_label, modelname)
    # pair, the original lazy function, and its positional and keyword args as
    # determined by extract_operation().

    def field_error(model_key, func, args, keywords):
        error_msg = (
            "The field %(field)s was declared with a lazy reference "
            "to '%(model)s', but %(model_error)s."
        )
        params = {
            'model': '.'.join(model_key),
            'field': keywords['field'],
            'model_error': app_model_error(model_key),
        }
        return Error(error_msg % params, obj=keywords['field'], id='fields.E307')

    def signal_connect_error(model_key, func, args, keywords):
        error_msg = (
            "%(receiver)s was connected to the '%(signal)s' signal with a "
            "lazy reference to the sender '%(model)s', but %(model_error)s."
        )
        receiver = args[0]
        # The receiver is either a function or an instance of class
        # defining a `__call__` method.
        if isinstance(receiver, types.FunctionType):
            description = "The function '%s'" % receiver.__name__
        elif isinstance(receiver, types.MethodType):
            description = "Bound method '%s.%s'" % (receiver.__self__.__class__.__name__, receiver.__name__)
        else:
            description = "An instance of class '%s'" % receiver.__class__.__name__
        signal_name = model_signals.get(func.__self__, 'unknown')
        params = {
            'model': '.'.join(model_key),
            'receiver': description,
            'signal': signal_name,
            'model_error': app_model_error(model_key),
        }
        return Error(error_msg % params, obj=receiver.__module__, id='signals.E001')

    def default_error(model_key, func, args, keywords):
        error_msg = "%(op)s contains a lazy reference to %(model)s, but %(model_error)s."
        params = {
            'op': func,
            'model': '.'.join(model_key),
            'model_error': app_model_error(model_key),
        }
        return Error(error_msg % params, obj=func, id='models.E022')

    # Maps common uses of lazy operations to corresponding error functions
    # defined above. If a key maps to None, no error will be produced.
    # default_error() will be used for usages that don't appear in this dict.
    known_lazy = {
        ('django.db.models.fields.related', 'resolve_related_class'): field_error,
        ('django.db.models.fields.related', 'set_managed'): None,
        ('django.dispatch.dispatcher', 'connect'): signal_connect_error,
    }

    def build_error(model_key, func, args, keywords):
        key = (func.__module__, func.__name__)
        error_fn = known_lazy.get(key, default_error)
        return error_fn(model_key, func, args, keywords) if error_fn else None

    return sorted(filter(None, (
        build_error(model_key, *extract_operation(func))
        for model_key in pending_models
        for func in apps._pending_operations[model_key]
    )), key=lambda error: error.msg)


@register(Tags.models)
def check_lazy_references(app_configs=None, **kwargs):
    return _check_lazy_references(apps)