如何檢索256個元素數組到mongodb？

Question

我有一個這樣的收集數據格式， 我現在傳輸和_id和行。發送_id：如何256元陣列 防爆返回4線1045，從（1045-256）回線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驅動程序中使用下面的代碼。

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

MongoDB中聚集管道使用$unwind，$match，$sort，$skip和$limit我們可以實現此

MongoDB的殼查詢

db.mycollection.aggregate（[{$展開： 「$指紋」}，{$匹配：{_id：4}}，{$排序：{行：1}} ，{$ skip：1045}，{$ limit：256}]）;

解釋 - 這怎麼查詢的工作

$放鬆 - 解開指紋陣列成單獨單據

$匹配 - 匹配所需要的文件，它在聚集管道移動到一個新的水平（過濾收集並有助於提供更快的速度）

$ sort - 將展開的文檔按升序排序（我們需要這個，否則我們可能會遺漏使用Skip和限制的幾個文檔）

$跳過 - 需要跳過這是我們已經提供的文件

$限制 - 限制結果的大小

例

讓我們有一個集合有3條記錄

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" 
    }] 
}] 

讓我們使用聚合管道獲得所需的結果

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" } } 

$放鬆 - unwinded收集和現在我們有15份文件

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" } } 

$跳過 - 要跳過的文件 $限額 - 要限制最終結果

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" } } 

請注意，我已經跳過1文件和限制我的結果爲2文件

希望它有助於！