mongodb:获取特定文档然后再获取其余文档的最佳方法

问题描述

让我说我有1000个文档，每个文档都有:

lets say I have 1000 documents where each one has:

user_id
text

现在，我想提取所有这些文档，但首先要从几个特定用户(给定用户ID数组)中提取文档，然后再提取其余所有文档.

Now, I would like to pull all those documents but first pull the documents from a few specific users (given an array of user ids) and then all the rest.

如果要在特定用户数组中存在user_id(使用范围传递该数组)，然后使用map reduce来创建一个新的权重内联属性，然后对该新属性进行排序.但是据我了解，您无法对map reduce进行排序.

I was thinking to use map reduce to create a new weight inline attribute if the user_id exists in the specific users array (using scope to pass the array) and then to sort that new attribute. But from what I could understand, you can not sort after map reduce.

任何人都有一个好的建议如何实现这一目标?任何建议都将受到欢迎.

Any one has a good suggestion how to pull this off? Any suggestion will be welcome.

谢谢！

推荐答案

这里没有很多细节，但是我可以给出一个示例案例进行考虑.考虑以下文档集:

Well there isn't a lot of detail here, but I can give a sample case for consideration. Consider the following set of documents:

{ "user" : "fred", "color" : "black" }
{ "user" : "bill", "color" : "blue" }
{ "user" : "ted", "color" : "red" }
{ "user" : "ted", "color" : "black" }
{ "user" : "fred", "color" : "blue" }
{ "user" : "bill", "color" : "red" }
{ "user" : "bill", "color" : "orange" }
{ "user" : "fred", "color" : "orange" }
{ "user" : "ted", "color" : "orange" }
{ "user" : "ally", "color" : "orange" }
{ "user" : "alice", "color" : "orange" }
{ "user" : "alice", "color" : "red" }
{ "user" : "bill", "color" : "purple" }

因此，假设您想冒泡将用户的项目比尔"和"ted"放在结果的顶部，然后将其他所有内容按user和color排序.您可以做的是通过 $ project 阶段运行文档汇总如下:

So suppose you want to bubble the items for the users "bill" and "ted" to the top of your results, then everything else sorted by the user and the color. What you can do is run the documents through a $project stage in aggregate, as follows:

db.bubble.aggregate([

    // Project selects the fields to show, and we add a weight value
    {$project: {
        _id: 0,
        "user": 1,
        "color": 1,
        "weight": {$cond:[
            {$or: [
                {$eq: ["$user","bill"]},
                {$eq: ["$user","ted"]}
            ]},
            1,
            0
         ]}
     }},

    // Then sort the results with the `weight` first, then `user` and `color`
    {$sort: { weight: -1, user: 1, color: 1 }}

])

因此，根据user是否与所需值之一匹配，有条件地为weight分配了一个值.不匹配的文档仅被赋予0值.

So what that does is conditionally assign a value to weight based on whether the user was matched to one of the required values. Documents that do not match are simply given a 0 value.

当我们将此经过修改的文档移至 $ sort 阶段，可以使用新的weight键对结果进行排序，以使加权"文档位于顶部，然后再进行其他操作.

When we move this modified document on to the $sort phase, the new weight key can be used to order the results so the "weighted" documents are on top, and anything else will then follow.

您可以对 $ project $ project 这样的重量.有关更多信息，请参见操作员参考:

There a quite a few things you can do to $project a weight in this way. See the operator reference for more information:

http://docs.mongodb.org/manual/reference/operator/aggregation /

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