循环关系对于RDF和对象模型非常常见。
Cyclic relationships are a common situation in RDF and in object models.
与关系数据库相比，面向对象模型支持更丰富的关系
Richer relationships allowed by object orientation that aren't supported by relational databases
对象Contact是模型所提供的ActiveRecord对象-关系映射。
The object Contact is the ActiveRecord object-relational mapping that the model provides.
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