DOC: https://docs.mongodb.com/manu...

collection（test）結構

{_id: Objectd("123456789"),category: ['apple_1','apple_2','banana_1','banana_2'] }

Question:

對test表的所有數據做category過濾，返回category中以apple開頭的元素

表原數據：

[ {_id: Objectd("id1"),category: ['apple_1','apple_2','banana_1','banana_2']},{_id: Objectd("id2"),category: ['apple_3','apple_4','banana_1','banana_2']}... ]

返回數據示例：

[ {_id: Objectd("id1"),category: ['apple_1','apple_2']},{_id: Objectd("id2"),category: ['apple_3','apple_4']}... ]

數據庫try：隨機構建test表

function getRandomArrayElements(arr, count) {var shuffled = arr.slice(0), i = arr.length, min = i - count, temp, index;while (i-- > min) {index = Math.floor((i + 1) * Math.random());temp = shuffled[index];shuffled[index] = shuffled[i];shuffled[i] = temp;}return shuffled.slice(min); } var temp = ['apple_1','apple_2','banana_3','banana_4','pear_5','pear_6','pear_7']; for(var i =0; i < 10; i ++){db.getCollection("test").insert({category:getRandomArrayElements(temp, Math.random()*7)}) }

Try 1:

db.test.find({},{'category':{'$elemMatch':{$regex: 'apple'} }})

返回：

[ {_id: Objectd("id1"),category: ['apple_1',]},{_id: Objectd("id2"),category: ['apple_3',]}... ] category只保留了符合過濾規則的第一個元素

Try 2:

db.test.aggregate({$unwind: '$category'},{$match: {category: {$regex: 'apple_'}}},//unwind，match順序不能換 )

返回：

[ {_id: Objectd("id1"),category: 'apple_1'},{_id: Objectd("id1"),category: 'apple_2'},{_id: Objectd("id2"),category: 'apple_3'},{_id: Objectd("id2"),category: 'apple_4'}... ] 將一個文檔拆分成多個文檔返回

Try 3（Solution）:

db.test.aggregate({$project: {"category":{$filter: {input: "$category",as: "cate",cond: {// category數組元素cate包含字符串'apple_'$eq: [ {$indexOfCP: ["$$cate", "apple_"]}, 0] }}}} })

返回：

[ {_id: Objectd("id1"),category: ['apple_1','apple_2']},{_id: Objectd("id2"),category: ['apple_3','apple_4']}... ]

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