Mongodb，使用 $lookup 进行聚合查询 [英] Mongodb, aggregate query with $lookup

问题描述

有两个收藏，标签和人物.

Got two collecetions, tags and persons.

标签模型:

{
  en: String,
  sv: String
}

人物模型:

{
  name: String,
  projects: [
    title: String,
    tags: [
      {
        type: Schema.ObjectId,
        ref: 'tag'
      }
    ]
  ]

}

我希望查询返回人员模型中正在使用的所有标签.所有文件.

I want query that returns all tags that is in use in the person model. All documents.

有点像

var query = mongoose.model('tag').find({...});

或者我应该以某种方式使用聚合方法吗?

Or should I somehow use the aggregate approach to this?

推荐答案

对于任何特定的个人文档，您可以使用 populate() 类似

For any particular person document, you can use the populate() function like

var query = mongoose.model("person").find({ "name": "foo" }).populate("projects.tags");

例如，如果您想搜索任何带有MongoDB"或Node JS"标签的人，您可以在 populate() 函数重载中包含查询选项:

And if you want to search for any persons that have any tag with 'MongoDB' or 'Node JS' for example, you can include the query option in the populate() function overload as:

var query = mongoose.model("person").find({ "name": "foo" }).populate({
    "path": "projects.tags",
    "match": { "en": { "$in": ["MongoDB", "Node JS"] } }
});

<小时>

如果您希望所有人的 "project.tags" 中存在所有标签，那么聚合框架就是要走的路.考虑在人员集合上运行此管道并使用 $查找 运算符对标签集合进行左连接:

---

If you want all tags existing in "project.tags" for all persons, then aggregation framework is the way to go. Consider running this pipeline on the person collection and uses the $lookup operator to do a left join on the tags collection:

mongoose.model('person').aggregate([
    { "$unwind": "$projects" },
    { "$unwind": "$projects.tags" },
    {
        "$lookup": {
            "from": "tags",
            "localField": "projects.tags",
            "foreignField": "_id",
            "as": "resultingTagsArray"
        }
    },
    { "$unwind": "$resultingTagsArray" },
    {
        "$group": {
            "_id": null,
            "allTags": { "$addToSet": "$resultingTagsArray" },
            "count": { "$sum": 1 }
        }
    }
 ]).exec(function(err, results){
    console.log(results);
 })

对于任何特定的人，然后应用 $match 管道作为过滤文档的第一步:

For any particular person then apply a $match pipeline as the first step to filter the documents:

mongoose.model('person').aggregate([
    { "$match": { "name": "foo" } },
    { "$unwind": "$projects" },
    { "$unwind": "$projects.tags" },
    {
        "$lookup": {
            "from": "tags",
            "localField": "projects.tags",
            "foreignField": "_id",
            "as": "resultingTagsArray"
        }
    },
    { "$unwind": "$resultingTagsArray" },
    {
        "$group": {
            "_id": null,
            "allTags": { "$addToSet": "$resultingTagsArray" },
            "count": { "$sum": 1 }
        }
    }
 ]).exec(function(err, results){
    console.log(results);
 })

<小时>

如果您使用不支持 $lookup 操作符将聚合结果填充为:

---

Another workaround if you are using MongoDB versions >= 2.6 or <= 3.0 which do not have support for the $lookup operator is to populate the results from the aggregation as:

mongoose.model('person').aggregate([
    { "$unwind": "$projects" },
    { "$unwind": "$projects.tags" },    
    {
        "$group": {
            "_id": null,
            "allTags": { "$addToSet": "$projects.tags" }
        }
    }
 ], function(err, result) {
    mongoose.model('person')
    .populate(result, { "path": "allTags" }, function(err, results) {
        if (err) throw err;
        console.log(JSON.stringify(results, undefined, 4 ));
    });
});

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