public function foreignField(string $foreignField) : self
    {
        $this->foreignField = $this->prepareFieldName($foreignField, $this->targetClass);
        return $this;
    }
public function foreignField(string $foreignField) : self
    {
        $this->foreignField = $this->prepareFieldName($foreignField, $this->targetClass);
        return $this;
    }
public function foreignField ( string $ foreignField ) : self { $ this -> foreignField = $ this -> prepareFieldName ( $ foreignField , $ this -> targetClass ) ; return $ this ; }
Specifies the field from the documents in the from collection.

$lookup performs an equality match on the foreignField to the localField
from the input documents. If a document in the from collection does not
contain the foreignField, the $lookup treats the value as null for
matching purposes.
Specifies the field from the documents in the from collection .