The basic Find query will be illustrated in other chapters. This article mainly focuses on the problems encountered and the solutions to make a record, hoping to reduce the compatriots who encounter such problems -_-!
The first part of the needs and problems
1.1 Data structure
1.1.1 Insert test data
db.hhw.insert({results: [
{ item: "a", qty: 26, tags: ["blank", "red"], dim_cm: [ 1, 10 ] },
{ item: "a", qty: 27, tags: ["blank", "red"], dim_cm: [ 15, 30 ] },
{ item: "a", qty: 28, tags: ["blank", "red"], dim_cm: [ 50, 70 ] },
{ item: "b", qty: 27, tags: ["blank", "red"], dim_cm: [ 80, 90 ] }
]
});
1.1.2 Get data, there is no ➕pretty() formatting here
> db.hhw.find()
{ "_id" : ObjectId("58b675a556341c2186667dd4"),
"results" : [
{ "item" : "a", "qty" : 26, "tags" : [ "blank", "red" ], "dim_cm" : [ 1, 10 ] },
{ "item" : "a", "qty" : 27, "tags" : [ "blank", "red" ], "dim_cm" : [ 15, 30 ] },
{ "item" : "a", "qty" : 28, "tags" : [ "blank", "red" ], "dim_cm" : [ 50, 70 ] },
{ "item" : "b", "qty" : 27, "tags" : [ "blank", "red" ], "dim_cm" : [ 80, 90 ] }
] }1.2 Requirements
A demand: the obtained results in the item value of
athe set of results.
The second part solves the problem
2.1 Method one to try -------- find clause $elemMatch
参考官网V3.4 eleMatch, 大致功能为投影操作符将限制查询返回的数组字段的内容只包含匹配elemMatch条件的数组元素。其实`$elemMatch专门用于查询数组Field中元素是否满足指定的条件。
> db.hhw.find({
results: { $elemMatch: { item: "a"}}})
#确实是全部数据
{ "_id" : ObjectId("58b675a556341c2186667dd4"),
"results" : [ { "item" : "a", "qty" : 26, "tags" : [ "blank", "red" ], "dim_cm" : [ 1, 10 ] },
{ "item" : "a", "qty" : 27, "tags" : [ "blank", "red" ], "dim_cm" : [ 15, 30 ] },
{ "item" : "a", "qty" : 28, "tags" : [ "blank", "red" ], "dim_cm" : [ 50, 70 ] },
{ "item" : "b", "qty" : 27, "tags" : [ "blank", "red" ], "dim_cm" : [ 80, 90 ] } ] }按照官网V3.4 eleMatch的实例,一直达不到预期的效果,按照的版本是v3.4.0, 文档参考的也是v3.4.0好奇!
解决的方法,参考了StackOverflow中, find之后的第一个参数设置为{}, 但是得写上.
> db.hhw.find({}, {
results: {
$elemMatch: { item: "a"}
}
})
#只返回了符合elemMatch的第一条数据
{ "_id" : ObjectId("58b675a556341c2186667dd4"),
"results" : [ { "item" : "a", "qty" : 26, "tags" : [ "blank", "red" ], "dim_cm" : [ 1, 10 ] } ] }注意：
1. 数组中元素是内嵌文档。 2. 如果多个元素匹配$elemMatch条件，操作符返回数组中第一个匹配条件的元素。
2.2 既然来了, 就操作一下$elemMatch吧
需求: 要查询 dim_cm 中满足至少存在一个元素大于等于10且小于20的
2.3 转到第三部分
第三部分 聚合aggregate
3.1 语法以及参数
>db.COLLECTION_NAME.aggregate(AGGREGATE_OPERATION)
>$project：修改输入文档的结构。可以用来重命名、增加或删除域，也可以用于创建计算结果以及嵌套文档。
$match：用于过滤数据，只输出符合条件的文档。
$match使用MongoDB的标准查询操作。
$limit：用来限制MongoDB聚合管道返回的文档数。
$skip：在聚合管道中跳过指定数量的文档，并返回余下的文档。
$unwind：将文档中的某一个数组类型字段拆分成多条，每条包含数组中的一个值。
$group：将集合中的文档分组，可用于统计结果。
$sort：将输入文档排序后输出。
$geoNear：输出接近某一地理位置的有序文档。参数太多,还是直接来上代码
*还是原来的问题,在此使用aggregate来解决
3.2代码如下
> db.hhw.aggregate(
{"$unwind": "$results"},
{"$match": {"results.item": "a"}},
{"$group":{ "_id":"$_id", "myResult":{ "$push": "$results"
}
}}
)#结果达到预期效果,三个a
{ "_id" : ObjectId("58b675a556341c2186667dd4"), "myResult" : [ { "item" : "a", "qty" : 26, "tags" : [ "blank", "red" ], "dim_cm" : [ 1, 10 ] }, { "item" : "a", "qty" : 27, "tags" : [ "blank", "red" ], "dim_cm" : [ 15, 30 ] }, { "item" : "a", "qty" : 28, "tags" : [ "blank", "red" ], "dim_cm" : [ 50, 70 ] } ] }3.3 接下来一步步拆分上边的三条语句
3.3.1 $unwind: 将文档中的某一个数组类型字段拆分成多条，每条包含数组中的一个值。类似于遍历,前后都有$符号
> db.hhw.aggregate({"$unwind": "$results"})
{ "_id" : ObjectId("58b675a556341c2186667dd4"), "results" : { "item" : "a", "qty" : 26, "tags" : [ "blank", "red" ], "dim_cm" : [ 1, 10 ] } }
{ "_id" : ObjectId("58b675a556341c2186667dd4"), "results" : { "item" : "a", "qty" : 27, "tags" : [ "blank", "red" ], "dim_cm" : [ 15, 30 ] } }
{ "_id" : ObjectId("58b675a556341c2186667dd4"), "results" : { "item" : "a", "qty" : 28, "tags" : [ "blank", "red" ], "dim_cm" : [ 50, 70 ] } }
{ "_id" : ObjectId("58b675a556341c2186667dd4"), "results" : { "item" : "b", "qty" : 27, "tags" : [ "blank", "red" ], "dim_cm" : [ 80, 90 ] } }
3.3.3 $match过滤数据
3.3.4 $group将集合中的文档分组，可用于统计结果
上边的例子我们将所有的字段统一返回, 假如此时我们只想返回item tags呢?
> db.hhw.aggregate(
{"$unwind": "$results"},
{"$match": {"results.item": "a"}},
{"$group":{ "_id":"$_id", "myResult":{ "$push": {"Item": "$results.item", "tags": "$results.tags"}
}
}}
)
{ "_id" : ObjectId("58b675a556341c2186667dd4"), "myResult" :
[ { "Item" : "a", "tags" : [ "blank", "red" ] },
{ "Item" : "a", "tags" : [ "blank", "red" ] },
{ "Item" : "a", "tags" : [ "blank", "red" ] }
] }
3.3.5 $project 限定结果集合
此时只有两个字段(_id, results), 只显示results
db.hhw.aggregate(
{ $project : {
_id : 0 ,
results : 1 ,
}}
);
{ "results" : [ { "item" : "a", "qty" : 26, "tags" : [ "blank", "red" ], "dim_cm" : [ 1, 10 ] }, { "item" : "a", "qty" : 27, "tags" : [ "blank", "red" ], "dim_cm" : [ 15, 30 ] }, { "item" : "a", "qty" : 28, "tags" : [ "blank", "red" ], "dim_cm" : [ 50, 70 ] }, { "item" : "b", "qty" : 27, "tags" : [ "blank", "red" ], "dim_cm" : [ 80, 90 ] } ] }
遇到的问题
X.1 在mongoose 中aggregate中match id 时出现错误的解决办法:
Product.aggregate(
{"$unwind": "$products"},
// 执行成功
{"$match": {"products.price": 9}},
// 执行失败
{"$match": {"products._id": "58b77e509219010a6eac48ed"}},#下边方法成功
{"$match": {"products._id": new mongoose.Types.ObjectId('58b77e509219010a6eac48ed')}},
function(err, result){ if (err) {
console.log(err);
}else {
res.send(result);
}
}
);
mongoose has 两个地方defined ObjectId, mongoose.schema.Types.ObjectIdand mongoose.Types.ObjectId , where only mongoose.Types.ObjectId to work.