Question

我有这样的集合数据格式，我现在传输和_id和行。如何返回256个元素数组例如：发送_id：4行1045，返回行（1045-256）到1045

{
  "_id" : 1,
  "fingerprint" : [ 
    {
        "line" : 1,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 2,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 3,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 4,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 5,
        "sub_finger" : "00000000"
    }, ...

Answer 1

您可以在mongodb java驱动程序中使用以下代码。

 MongoClient client = new MongoClient("localhost", 27017);
 MongoDatabase database = client.getDatabase("test");
 MongoCollection<Document> collection = database.getCollection("collection");
 Document result = collection.find(Filters.eq("_id", 4)).projection(Projections.slice("fingerprint", 1045-256, 256)).first()

Answer 2

在mongodb聚合管道中使用$unwind，$match，$sort，$skip和$limit，我们可以实现此目标

MongoDB shell Query

db.mycollection.aggregate（[{$ unwind：＆＃34; $ fingerprint＆＃34;}，{$ match：{_ id：4}}，{$ sort：{line：1}}，{$ skip ：1045}，{$ limit：256}]）;

说明 - 此查询的工作原理

$ unwind - 将指纹数据展开为单个文档

$ match - 匹配所需的文档并将其移至聚合管道中的下一级别（过滤集合并帮助更快地交付）

$ sort - 按升序对未展开的文档进行排序（我们需要这个，否则我们可能会错过很少使用Skip和限制的文档）

$ skip - 需要跳过我们已经呈现的文件

$ limit - 限制结果的大小

实施例

让我们有一个包含3条记录的集合

db.mycollection.find()

[{
  "_id" : 1,
  "fingerprint" : [ 
    {
        "line" : 1,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 2,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 3,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 4,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 5,
        "sub_finger" : "00000000"
    }]
},
{
  "_id" : 2,
  "fingerprint" : [ 
    {
        "line" : 1,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 2,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 3,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 4,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 5,
        "sub_finger" : "00000000"
    }]
},
{
  "_id" : 3,
  "fingerprint" : [ 
    {
        "line" : 1,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 2,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 3,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 4,
        "sub_finger" : "00000000"
    }, 
    {
        "line" : 5,
        "sub_finger" : "00000000"
    }]
}]

让我们使用聚合管道来获得所需的结果

db.mycollection.aggregate([{$unwind:"$fingerprint"}])

{ "_id" : 1, "fingerprint" : { "line" : 1, "sub_finger" : "00000000" } }
{ "_id" : 1, "fingerprint" : { "line" : 2, "sub_finger" : "00000000" } }
{ "_id" : 1, "fingerprint" : { "line" : 3, "sub_finger" : "00000000" } }
{ "_id" : 1, "fingerprint" : { "line" : 4, "sub_finger" : "00000000" } }
{ "_id" : 1, "fingerprint" : { "line" : 5, "sub_finger" : "00000000" } }
{ "_id" : 2, "fingerprint" : { "line" : 1, "sub_finger" : "00000000" } }
{ "_id" : 2, "fingerprint" : { "line" : 2, "sub_finger" : "00000000" } }
{ "_id" : 2, "fingerprint" : { "line" : 3, "sub_finger" : "00000000" } }
{ "_id" : 2, "fingerprint" : { "line" : 4, "sub_finger" : "00000000" } }
{ "_id" : 2, "fingerprint" : { "line" : 5, "sub_finger" : "00000000" } }
{ "_id" : 3, "fingerprint" : { "line" : 1, "sub_finger" : "00000000" } }
{ "_id" : 3, "fingerprint" : { "line" : 2, "sub_finger" : "00000000" } }
{ "_id" : 3, "fingerprint" : { "line" : 3, "sub_finger" : "00000000" } }
{ "_id" : 3, "fingerprint" : { "line" : 4, "sub_finger" : "00000000" } }
{ "_id" : 3, "fingerprint" : { "line" : 5, "sub_finger" : "00000000" } }

$ unwind - 展开集合，现在我们有15个文件

db.mycollection.aggregate([{$unwind:"$fingerprint"}, {$match: {_id:1}}])

Note: Pass the respective document id in the $match query

{ "_id" : 1, "fingerprint" : { "line" : 1, "sub_finger" : "00000000" } }
{ "_id" : 1, "fingerprint" : { "line" : 2, "sub_finger" : "00000000" } }
{ "_id" : 1, "fingerprint" : { "line" : 3, "sub_finger" : "00000000" } }
{ "_id" : 1, "fingerprint" : { "line" : 4, "sub_finger" : "00000000" } }
{ "_id" : 1, "fingerprint" : { "line" : 5, "sub_finger" : "00000000" } }

$ skip - 要删除文档 $ limit - 限制最终结果

db.mycollection.aggregate([{$unwind:"$fingerprint"}, {$match: {_id:1}},{$sort: {line:1}}, {$skip:1}, {$limit:2}]);

{ "_id" : 1, "fingerprint" : { "line" : 2, "sub_finger" : "00000000" } }
{ "_id" : 1, "fingerprint" : { "line" : 3, "sub_finger" : "00000000" } }

请注意，我已跳过1个文档并将结果限制为2个文档

希望它有帮助！

如何检索256元素数组到mongodb？

2 个答案: