mongoDB - 基于返回结果数量的不同查询条件

Question

我有这样一个文档-

{
type: "One",
details: "This is one",
at: "some place, where one is relevant"
}

类似架构的其他文档可以具有相同的“类型”，但具有不同的“详细信息”、“at”等。可以有多个“类型”。

现在，我想编写一个查询来返回一定数量(上限，比如 5)匹配特定“类型”的文档(我可以使用 limit 和 $in ) 如果结果包含的文档少于 5 个，则可以省略“类型”标准。

例如，如果我只允许“一个”和“两个”作为“类型”并且我使用 5 的限制，那么如果结果的数量小于 5(比如 2)，它应该返回我那些类型为“一”和“二”的文档(即 2 个文档)和另外 3 个未查看其“类型”的文档。

我希望这是有道理的!

Answer 1

如果您不喜欢使用脚本，我可以看到的选项是使用 aggregate 并引入额外的字段 weight 例如向上移动匹配的文档然后排序权重和限制总结果:

db.test.aggregate({ 
  $project: {
    type: 1, 
    details: 1, 
    at: 1, 
    weight: { 
      $cond: [ 
        { "$or": [  
          {$eq: ["$type", "One"] }, 
          {$eq: ["$type", "Two"] } 
        ] }, 
      0, 1] } 
    } },  
    {$sort: {weight: 1}},
    { $limit : 5 }
);

关于这个例子的注释。为了简单起见，我将 $in 替换为 $or 中的几个相等项。如果您不希望在最终结果中包含权重，您可以通过在聚合管道中应用另一个投影来移除它。

测试数据库:

> db.test.find({})
{ "_id" : ObjectId("58067329a7518db4d3d2e973"), "type" : "One", "details" : "This is one", "at" : "some place, where one is relevant" }
{ "_id" : ObjectId("5806733da7518db4d3d2e974"), "type" : "Two", "details" : "This is two", "at" : "some place, where one is relevant" }
{ "_id" : ObjectId("5806734ba7518db4d3d2e975"), "type" : "Three", "details" : "This is three", "at" : "some place, where one is relevant" }
{ "_id" : ObjectId("58067374a7518db4d3d2e976"), "type" : "Four", "details" : "This is four", "at" : "some place, where one is relevant" }
{ "_id" : ObjectId("58067381a7518db4d3d2e977"), "type" : "Five", "details" : "This is five", "at" : "some place, where one is relevant" }
{ "_id" : ObjectId("5806738fa7518db4d3d2e978"), "type" : "Six", "details" : "This is six", "at" : "some place, where one is relevant" }
{ "_id" : ObjectId("580673cfa7518db4d3d2e979"), "type" : "Seven", "details" : "This is seven", "at" : "some place, where one is relevant" }    

结果:

> db.test.aggregate({ $project: { type: 1, details: 1, at: 1, weight: { $cond: [ { "$or": [  {$eq: ["$type", "One"] }, {$eq: ["$type", "Two"] } ] }, 0, 1] } } },  {$sort: {weight: 1}}, { $limit : 5 }, {$project: {type: 1, details: 1, at: 1} });
{ "_id" : ObjectId("58067329a7518db4d3d2e973"), "type" : "One", "details" : "This is one", "at" : "some place, where one is relevant" }
{ "_id" : ObjectId("5806733da7518db4d3d2e974"), "type" : "Two", "details" : "This is two", "at" : "some place, where one is relevant" }
{ "_id" : ObjectId("58067374a7518db4d3d2e976"), "type" : "Four", "details" : "This is four", "at" : "some place, where one is relevant" }
{ "_id" : ObjectId("58067381a7518db4d3d2e977"), "type" : "Five", "details" : "This is five", "at" : "some place, where one is relevant" }
{ "_id" : ObjectId("5806734ba7518db4d3d2e975"), "type" : "Three", "details" : "This is three", "at" : "some place, where one is relevant" }