  
      if pk:
          pk_field = obj._meta.pk
          while pk_field.rel:
              pk_field = pk_field.rel.to._meta.pk
          include.append(pk_field.name)
  
      if slug_field:
 
</s>
        # Pending deprecation
  
      if pk:
        # Pending deprecation
          pk_field = obj._meta.pk
          while pk_field.rel:
              pk_field = pk_field.rel.to._meta.pk
          include.append(pk_field.name)
  
      if slug_field:
keep add keep keep keep keep keep keep
 introduce lookup_field and add pendingdeprecationwarnings
rest_framework/mixins.py