使用jacksonMapper时，以双向关系丢失子数据 [英] Losing the child data in a bi-directional relationship when using jacksonMapper

问题描述

我有两个类，如下所示，双向多对多关系：

I have two classes as shown below in a bi-directional Many to Many relationship:

  Parent implements Serializable{

        @ManytoMany(//declaration for join table)
        @JsonBackReference
        @com.fasterxml.jackson.annotation.JsonIgnore
        Set <Child> childSet;

    }
    Child implements Serializable{
    @ManytoMany(//declaration for join table)
    @JsonManagedReference
    @com.fasterxml.jackson.annotation.JsonIgnore
    Set <Parent> parentSet;
    // other getter and setters
    }

我打电话给我DAO得到一个特定的父母。与父详细信息一起，我想获取父项的子项。这样的事情：

I make a call in my DAO to get a particular parent. Alongwith the parent details I want to fetch the children of the parent. Something like this:

   Hibernate.initialize(parent.getChildSet()); //this works perfectly
// and I get the details of parent along with the children in my DAO call.

但是当我在业务服务中执行以下操作时将数据返回给控制器时，子项被省略来自父json字符串。

But when I do the below in my business service while returning the data to the controller the children are omitted from the parent json string.

jacksonMapper.writeValueAsString(parent); 

所以我在Parent类中移除了@JsonIgnore上的Child属性，认为jackson可能会理解这些字段是写入字符串时不要忽略，如下所示。但它仍然忽略了它们！ :(

So i removed the @JsonIgnore on Child attribute inside Parent class thinking that jackson might understand that these fields are not to be ignored while writing to a string as shown below. But it still does ignore them! :(

 Parent implements Serializable{

    @ManytoMany(//declaration for join table)
    @JsonBackReference
    //@com.fasterxml.jackson.annotation.JsonIgnore
    Set <Child> childSet;

}

我知道哪里可能出错？

推荐答案

我一直无法找到原因。同时我选择了一种解决方法。我正在对DB进行两次单独的调用。一次是先获取父级，然后是第二次根据获取的内容获取子级。 parentId。

I have not been able to find out why this is happening. Meanwhile I have opted for a workaround. I am making two separate calls to DB. One to fetch the parent first and then second to fetch the child based on the fetched parentId.

或者，我可以同时在服务上同时进行数据库调用，并在将JSON发送到ui之前将其作为复杂字符串进行准备：

Alternatively, I can make both the DB calls at the service same time and prepare the JSON as a complex string before sending it to the ui:

complex:{
parent:parent,
child:child
}

在任何一种情况下，它都是一种解决方法。理想的解决方案是仅在父映射中删除@JsonIgnore儿童班的一面。但不知何故，这似乎不起作用。如果我发现为什么理想的解决方案无效，我会发帖！

In either case, it is a workaround. The ideal solution is just remove@JsonIgnore in the mapping only from the parent side for the child class. But somehow that doesn't seem to work. I'll post in case I find why the "ideal" solution is not working!

理想解决方案2016年8月15日更新为答案印度独立日：

问题在于映射：

Parent implements Serializable{

        @ManytoMany(//declaration for join table)
        @JsonBackReference
        @com.fasterxml.jackson.annotation.JsonIgnore
        Set <Child> childSet;

    }

当你说@JsonBackReference它实际上意味着忽略这个字段而写Json，也就是说，

When you say @JsonBackReference it actually means ignore this field while writing the Json, i.e. to say,

@JsonBackReference< - > @JsonIgnore

@JsonBackReference <-> @JsonIgnore

因此，在序列化父项时，将省略子项。使用ORM映射，将注释设置为单面而非双面是最佳实践。通过这种方式，您可以在获取数据时避免大量不必要的异常，其次，保持业务代码清洁。

Hence the child is omitted when the parent is serialized. With ORM mappings it's always a best practice to have the annotations one sided rather than double sided. In that way, you can avoid a lot of unwanted exceptions while fetching the data and secondly, keep your business code clean.

JsonManagedReference vs JsonBackReference

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