"""
Helper functions for returning the field information that is associated
with a model class. This includes returning all the forward and reverse
relationships and their associated metadata.
"""
from collections import namedtuple
from django.db import models
from django.utils import six
from django.utils.datastructures import SortedDict
import inspect

FieldInfo = namedtuple('FieldResult', ['pk', 'fields', 'forward_relations', 'reverse_relations'])
RelationInfo = namedtuple('RelationInfo', ['field', 'related', 'to_many', 'has_through_model'])


def _resolve_model(obj):
    """
    Resolve supplied `obj` to a Django model class.

    `obj` must be a Django model class itself, or a string
    representation of one.  Useful in situtations like GH #1225 where
    Django may not have resolved a string-based reference to a model in
    another model's foreign key definition.

    String representations should have the format:
        'appname.ModelName'
    """
    if isinstance(obj, six.string_types) and len(obj.split('.')) == 2:
        app_name, model_name = obj.split('.')
        return models.get_model(app_name, model_name)
    elif inspect.isclass(obj) and issubclass(obj, models.Model):
        return obj
    raise ValueError("{0} is not a Django model".format(obj))


def get_field_info(model):
    """
    Given a model class, returns a `FieldInfo` instance containing metadata
    about the various field types on the model.
    """
    opts = model._meta.concrete_model._meta

    # Deal with the primary key.
    pk = opts.pk
    while pk.rel and pk.rel.parent_link:
        # If model is a child via multitable inheritance, use parent's pk.
        pk = pk.rel.to._meta.pk

    # Deal with regular fields.
    fields = SortedDict()
    for field in [field for field in opts.fields if field.serialize and not field.rel]:
        fields[field.name] = field

    # Deal with forward relationships.
    forward_relations = SortedDict()
    for field in [field for field in opts.fields if field.serialize and field.rel]:
        forward_relations[field.name] = RelationInfo(
            field=field,
            related=_resolve_model(field.rel.to),
            to_many=False,
            has_through_model=False
        )

    # Deal with forward many-to-many relationships.
    for field in [field for field in opts.many_to_many if field.serialize]:
        forward_relations[field.name] = RelationInfo(
            field=field,
            related=_resolve_model(field.rel.to),
            to_many=True,
            has_through_model=(
                not field.rel.through._meta.auto_created
            )
        )

    # Deal with reverse relationships.
    reverse_relations = SortedDict()
    for relation in opts.get_all_related_objects():
        accessor_name = relation.get_accessor_name()
        reverse_relations[accessor_name] = RelationInfo(
            field=None,
            related=relation.model,
            to_many=relation.field.rel.multiple,
            has_through_model=False
        )

    # Deal with reverse many-to-many relationships.
    for relation in opts.get_all_related_many_to_many_objects():
        accessor_name = relation.get_accessor_name()
        reverse_relations[accessor_name] = RelationInfo(
            field=None,
            related=relation.model,
            to_many=True,
            has_through_model=(
                hasattr(relation.field.rel, 'through') and
                not relation.field.rel.through._meta.auto_created
            )
        )

    return FieldInfo(pk, fields, forward_relations, reverse_relations)