Is there a way to save instance with many relations?

I have two models ModelA and ModelB. The ModelA has many relation to ModelB. In ModelB i have field model_a_id. Getting relations for both models works fine, so relations os ok.

I try to store new instance of ModelA via api request with the next values:
```
...
somefield=somevalue
model_bs[1][somefield]=somevalue
model_bs[2][somefield]=somevalue
...
```
But got an exception:
```
(1/1) BadMethodCallException
Call to undefined method Illuminate\Database\Query\Builder::getForeignKey()
```

I know how to store it by native eloquent method, but is there are some way to do it via repository?

Thanx!