title: peewee instance matching query does not exist
body: <p>I have the following code where I'm querying my peewee database. I'm getting an error in the for loop on the line that has term.sets_term_id. This is the error TermsDoesNotExist: Instance matching query does not exist: If I do vars(term) then sets_terms_id is an option. Why am I getting that error and how do I fix it?</p>
  
  <pre><code>def get_api_response(id):
      response = {}
      print("id is " + str(id))
      try:
          sets = models.Sets.select().where(models.Sets.user_id == id)
  
      except models.DoesNotExist:
          return json.loads({'error' : 'that set does not exist'})
      else:
          print("it was successful")
          for term in sets:
              print(term.sets_term_id)
  </code></pre>
  
  <p>Here are my Sets and Terms models</p>
  
  <pre><code>class Terms(UserMixin, BaseModel):
      term_id = CharField()
      sets_id = CharField()
      rank = IntegerField()
      term = TextField()
      definition = TextField()
  
      @classmethod
      def include_term(cls, set_id, term_id, definition, rank, term, **kwards):
          try:
              cls.select().where(cls.term_id == term_id).get()
          except cls.DoesNotExist:
              print("putting term into db")
              concept = cls(
                  sets_id = set_id,
                  term_id = term_id,
                  definition = definition,
                  rank = rank,
                  term= term)
              concept.save()
              print(concept.term)
              print("term saved to db")
              return concept
          else:
              raise Exception("Term with that id already exists")
  
  class Sets(UserMixin, BaseModel):
      user_id = CharField()
      name_set = CharField()
      #created_date = DateField()
      #modified_date = DateField()
      #published_date = DateField()
      sets_term_id = ForeignKeyField(Terms, to_field="sets_id")
  
      @classmethod
      def include_set(cls, user_id, name_set, sets_term_id, **kwards):
          try:
              cls.select().where(
                  (cls.name_set == name_set) | (cls.sets_term_id == sets_term_id)
              ).get()
          except cls.DoesNotExist:
              print("putting set into db")
              quizlet_set = cls(
                  user_id = user_id, 
                  name_set = name_set,
                  sets_term_id = sets_term_id)
              print(quizlet_set.name_set)
              quizlet_set.save()
              print("set saved in db")
  
              return quizlet_set
          else:
              raise Exception("Set with that name already exists")  
  </code></pre>