```
[2] pry(#<WikisController>)> params.require(:wiki).permit(:title, :body, :is_private, :collaborator_ids)
=> {"title"=>"MyString", "body"=>"MyText"}
[3] pry(#<WikisController>)> params.require(:wiki).permit(:title, :body, :is_private, :collaborator_ids => [])
=> {"title"=>"MyString", "body"=>"MyText", "collaborator_ids"=>["1"]}
```
for some reason, we need to indicate it's an array or it will not be added