Related models not added when using full=True

[ukch]

From #564. Similar to #307.

Consider an example app with two models, Continent and Country, where a continent may have many countries, and a country must belong to one continent. The resources are set up as follows:

class ContinentResource(ModelResource):
    countries = fields.ToManyField("pietest.api.CountryResource", "countries", full=True, related_name="continent")

    class Meta:
        queryset = Continent.objects.all()
        include_resource_uri = False
        excludes = ["id"]
        authorization = Authorization()  # for testing only


class CountryResource(ModelResource):

    class Meta:
        queryset = Country.objects.all()
        include_resource_uri = False
        excludes = ["id"]

The use of full=True should mean that I am able to post JSON such as the following, in order to create one continent and one country beneath it:

{
   "name": "Europe", 
   "countries": [
        {
            "name": "France"
        }
    ]
}

However, on posting this data the continent is successfully created, but then the process fails when creating the country, and the following traeback is returned:

Traceback (most recent call last):

  File "lib/tastypie/resources.py", line 192, in wrapper
    response = callback(request, *args, **kwargs)

  File "lib/tastypie/resources.py", line 397, in dispatch_list
    return self.dispatch('list', request, **kwargs)

  File "lib/tastypie/resources.py", line 427, in dispatch
    response = method(request, **kwargs)

  File "lib/tastypie/resources.py", line 1165, in post_list
    updated_bundle = self.obj_create(bundle, request=request, **self.remove_api_resource_names(kwargs))

  File "lib/tastypie/resources.py", line 1784, in obj_create
    self.save_m2m(m2m_bundle)

  File "lib/tastypie/resources.py", line 1951, in save_m2m
    related_bundle.obj.save()

  File "lib/django/db/models/base.py", line 460, in save
    self.save_base(using=using, force_insert=force_insert, force_update=force_update)

  File "lib/django/db/models/base.py", line 553, in save_base
    result = manager._insert(values, return_id=update_pk, using=using)

  File "lib/django/db/models/manager.py", line 195, in _insert
    return insert_query(self.model, values, **kwargs)

  File "lib/django/db/models/query.py", line 1436, in insert_query
    return query.get_compiler(using=using).execute_sql(return_id)

  File "lib/django/db/models/sql/compiler.py", line 791, in execute_sql
    cursor = super(SQLInsertCompiler, self).execute_sql(None)

  File "lib/django/db/models/sql/compiler.py", line 735, in execute_sql
    cursor.execute(sql, params)

  File "lib/django/db/backends/util.py", line 34, in execute
    return self.cursor.execute(sql, params)

  File "lib/django/db/backends/postgresql_psycopg2/base.py", line 44, in execute
    return self.cursor.execute(query, args)

IntegrityError: null value in column "continent_id" violates not-null constraint

This issue can be worked around by specifying the backwards relation (à la this Stackoverflow post), but this then causes the related issue (or non-issue) #564.