I use this ruby driver for mongo : <https://github.com/mongodb/mongo-ruby-driver>

I have many documents in a collection like this : 

```
{
  "_productId" => 10
  ....
}
{
  "_productId" => 10
  ....
}
{
  "_productId" => 12
  ....
}

```

What is the request to get all documents with a distinct "\_productId", I mean I want to take the first and the last collection item in this exemple.