Django模型：用于创建另一个模型的实例的集合/列表的字段类型

Consider these three models:

考虑这三个模型:

class Statement(models.Model):
    statement_id = models.CharField(max_length=200)
    title = models.CharField(max_length=200)
    statement-keywords = ?
    statement-contexts = ?

class Keyword(models.Model):
    word = models.CharField(max_length=200)
    statement = models.ManyToManyField(Statement)
    def __str__(self):
        return self.word

@python_2_unicode_compatible
class Context(models.Model):
    context_word = models.CharField(max_length=200)
    keyword = models.ManyToManyField(Keyword)
    statement = models.ManyToManyField(Statement)
    def __str__(self):
       return self.context_word
class