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

collection（test）结构

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

Question:

对test表的所有数据做category过滤，返回category中以apple开头的元素

表原数据：

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

返回数据示例：

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

数据库try：随机构建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只保留了符合过滤规则的第一个元素

Try 2:

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

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'}...
]

将一个文档拆分成多个文档返回

Try 3（Solution）:

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

返回：

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