仅向数组添加唯一性，并在更新时保持字段计数 [英] Add Unique only to array and keep field count on update

问题描述

我将一个项目数组(stringId)存储在集合中.此数组中的所有元素都必须是唯一的.因此，我使用$ addToSet推送项目.

I store in a collection an array of item (stringId). All element in this array must be unique. So I use $addToSet to push my item.

但是，我也想在同一请求中设置字段中数组的大小:

But, I also would like to set in the same request the size of my array in a field :

{
  unique_array: ['12', '20', '18'],
  size_of_array: 3
}

=>添加到设置15

=> Add to set 15

{
  unique_array: ['12', '20', '18', '15'], => Add to set
  size_of_array: 4 => Incremented
}

=>添加到设置18

=> Add to set 18

{
  unique_array: ['12', '20', '18', '15'], => Already in the set
  size_of_array: 4 => Not incremented
}

谢谢！

推荐答案

对于这种类型的操作，您不应使用 $addToSet 当然，由于 $inc 都会发生是否添加到数组(设置").

For this type of operation you should not use $addToSet since of course the $inc would happen regardless of whether anything was added to the array ( "set" ) or not.

相反，请使用 $ne 运算符测试阵列在查询中:

Instead, test the arrays with the $ne operator in the query:

db.collection.update(
    { "unique_array": { "$ne": 18 } },    <-- existing element
    { 
        "$push": { "unique_array": 18 },
        "$inc": { "size_of_array": 1 }
    }
)

删除数组成员的方法也一样，但是当然这次您测试是否相等:

The same goes for removing array members, but of course this time you test for the presence with equality:

db.collection.update(
    { "unique_array": 18 },    <-- existing element
    { 
        "$pull": { "unique_array": 18 },
        "$inc": { "size_of_array": -1 }
    }
)

由于查询条件需要匹配，因此添加时如果数组元素已经存在，则不存在匹配，因此 $pull 的情况也是如此不在数组中.

Since the query condition needs to match, if the array element was already present when adding then there is no match and netiher the $push or $inc operations are run. And the same is true for the $pull case where the element is not present in the array.

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