Has_many is a static method call for which the symbol: items is a parameter.
has_many是一个静态方法调用，符号：items是它的参数。
An assignment operator shall be implemented by a non-static member function with exactly one parameter.
赋值操作符应该实现为带一个参数的非静态成员函数。
An assignment operator shall be implemented by a non-static member function with exactly one parameter.
赋值操作符应该实现为带一个参数的非静态成员函数。
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