如何在MongoDB中存儲/檢索樹結構？

Question

首先我知道這篇文章https://docs.mongodb.com/v3.2/applications/data-models-tree-structures/。

但是我需要檢索多層次的樹結構。 例如，對於數據：

{ doc: "a", children: ["b", "c"] } 
{ doc: "b", children: ["d"] } 
{ doc: "c", children: ["e"] } 
{ doc: "d", children: [] } 
{ doc: "e", children: ["f"] } 
{ doc: "f", children: [] } 

我需要檢索文檔的 「」 全樹：

a 
b 
    d 
c 
    e 
    f 

是否有可能在MongoDB中？

Answer 1

因爲MongoDB的3.4可以使用$ graphLookup

該階段將一個文檔的connectFromField與集合中其他文檔的connectToField相匹配。然後，對於任何匹配的文件中，$ graphLookup使用匹配的文件，以匹配到的connectFromField其他文件的connectToField，並一直持續到沒有新的文件都遇到或到指定深度

https://docs.mongodb.com/master/release-notes/3.4-reference/#pipe._S_graphLookup

Answer 2

您可以嘗試黑客使用的$lookup和$unwind運營商如下聚合管道：

db.collection.aggregate([ 
    { 
     "$unwind": { 
      "path": "$children", 
      "preserveNullAndEmptyArrays": true 
     } 
    }, 
    { 
     "$lookup": { 
      "from": "test", 
      "localField": "children", 
      "foreignField": "doc", 
      "as": "child" 
     } 
    }, 
    { 
     "$unwind": { 
      "path": "$child", 
      "preserveNullAndEmptyArrays": true 
     } 
    }, 
    { 
     "$unwind": { 
      "path": "$child.children", 
      "preserveNullAndEmptyArrays": true 
     } 
    },  
    { 
     "$lookup": { 
      "from": "test", 
      "localField": "child.children", 
      "foreignField": "doc", 
      "as": "child.child" 
     } 
    }, 
    { 
     "$unwind": { 
      "path": "$child.child", 
      "preserveNullAndEmptyArrays": true 
     } 
    }, 
    { 
     "$unwind": { 
      "path": "$child.child.children", 
      "preserveNullAndEmptyArrays": true 
     } 
    }, 
    { 
     "$lookup": { 
      "from": "test", 
      "localField": "child.child.children", 
      "foreignField": "doc", 
      "as": "child.child.child" 
     } 
    }, 
    { 
     "$unwind": { 
      "path": "$child.child.child", 
      "preserveNullAndEmptyArrays": true 
     } 
    }, 
    { 
     "$unwind": { 
      "path": "$child.child.child.children", 
      "preserveNullAndEmptyArrays": true 
     } 
    },  
    { 
     "$group": { 
      "_id": "$doc", 
      "children": { "$push": "$child" } 
     } 
    }, 
    { "$match": { "_id": "a" } } 
]) 

樣本輸出

{ 
    "_id" : "a", 
    "children" : [ 
     { 
      "_id" : ObjectId("58204f0cd3cda4b4b1adadfb"), 
      "doc" : "b", 
      "children" : "d", 
      "child" : { 
       "_id" : ObjectId("58204f0cd3cda4b4b1adadfd"), 
       "doc" : "d" 
      } 
     }, 
     { 
      "_id" : ObjectId("58204f0cd3cda4b4b1adadfc"), 
      "doc" : "c", 
      "children" : "e", 
      "child" : { 
       "_id" : ObjectId("58204f0cd3cda4b4b1adadfe"), 
       "doc" : "e", 
       "children" : "f", 
       "child" : { 
        "_id" : ObjectId("58204f0cd3cda4b4b1adadff"), 
        "doc" : "f" 
       } 
      } 
     } 
    ] 
}