Django 2.0 mixer.blend fails to populate model that has foreign key

Hi,

Here are my models:

```
class Instrument(models.Model):
stock_code = models.CharField(max_length=50)
asset_type = models.ForeignKey(AssetType, on_delete=models.DO_NOTHING)
data = JSONField()

def __str__(self):
return self.stock_code
```

```
class AssetType(models.Model):
code = models.CharField(max_length=5)
name = models.CharField(max_length=25)

def __str__(self):
return self.name
```

When I try to test Instrument model:

```
class TestInstrument:
def test_model(self):

obj = mixer.blend(Instrument)

assert obj.pk == 1, "Should save an Instrument instance"
```

I receive following error:

```
_____________________________________________________________________________ TestInstrument.test_model _____________________________________________________________________________

self = , value =

def to_python(self, value):
if value is None:
return value
try:
> return int(value)
E TypeError: int() argument must be a string, a bytes-like object or a number, not 'AssetType'

../venv/lib/python3.6/site-packages/django/db/models/fields/__init__.py:940: TypeError

During handling of the above exception, another exception occurred:

self =

def test_model(self):

> obj = mixer.blend(Instrument)

strategy/tests/test_models.py:34:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../venv/lib/python3.6/site-packages/mixer/main.py:568: in blend
return type_mixer.blend(**values)
../venv/lib/python3.6/site-packages/mixer/main.py:116: in blend
for name, value in defaults.items()
../venv/lib/python3.6/site-packages/mixer/main.py:116: in
for name, value in defaults.items()
../venv/lib/python3.6/site-packages/mixer/mix_types.py:222: in gen_value
return type_mixer.gen_field(field)
../venv/lib/python3.6/site-packages/mixer/backend/django.py:270: in gen_field
return super(TypeMixer, self).gen_field(field)
../venv/lib/python3.6/site-packages/mixer/main.py:193: in gen_field
return self.gen_value(field.name, field, unique=unique)
../venv/lib/python3.6/site-packages/mixer/main.py:261: in gen_value
return self.get_value(field_name, value)
../venv/lib/python3.6/site-packages/mixer/backend/django.py:218: in get_value
return self._get_value(name, value, field)
../venv/lib/python3.6/site-packages/mixer/backend/django.py:233: in _get_value
value = field.scheme.to_python(value)
../venv/lib/python3.6/site-packages/django/db/models/fields/related.py:874: in to_python
return self.target_field.to_python(value)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = , value =

def to_python(self, value):
if value is None:
return value
try:
return int(value)
except (TypeError, ValueError):
raise exceptions.ValidationError(
self.error_messages['invalid'],
code='invalid',
> params={'value': value},
)
E django.core.exceptions.ValidationError: ["'Jared Smith' value must be an integer."]

../venv/lib/python3.6/site-packages/django/db/models/fields/__init__.py:945: ValidationError
```

What is wrong with my models?

Thanks

Reproduce the failure with the reported Instrument and AssetType models and the test_model test. Start in mixer/backend/django.py, following the call through mixer/main.py and mixer/mix_types.py. Done means mixer.blend(Instrument) completes without the foreign-key validation error and the test passes.

Written by the indexing model from the issue text.