Use $expr and $where selectors in MongoDB query documents

We have introduced before using comparison selectors, logical selectors, element selectors, and array selectors to query documents. If you need further information, you can refer to:

Comparison selectors and logical selectors are used in MongoDB query documents https://blog.csdn.net/m1729339749/article/details/129965699 Element selectors and array selectors are used in MongoDB query documents https://blog.csdn.net /m1729339749/article/details/129971708 In this article, we introduce the use of $expr and $where selectors to query documents:

1. Preparation

Initialize course grade data

db.subjectScores.insertMany([
    { "_id": 1, "name": "张三", "subject": "eng", "score": 80 },
    { "_id": 2, "name": "李四", "subject": "eng", "score": 60 },
    { "_id": 3, "name": "王五", "subject": "eng", "score": 90 },
    { "_id": 4, "name": "张三", "subject": "math", "score": 70 },
    { "_id": 5, "name": "李四", "subject": "math", "score": 90 },
    { "_id": 6, "name": "王五", "subject": "math", "score": 50 },
    { "_id": 7, "name": "张三", "subject": "physics", "score": 80 },
    { "_id": 8, "name": "李四", "subject": "physics", "score": 60 },
    { "_id": 9, "name": "王五", "subject": "physics", "score": 70 }
])

2. Use aggregation expressions to query documents ($expr)

grammar:

{ $expr: { <expression> } }

Allows querying documents using aggregate expressions in query statements

If you don't know much about aggregation expressions , you can refer to:

The use of MongoDB aggregation pipeline and the introduction of aggregation expressions https://blog.csdn.net/m1729339749/article/details/130034020

Example: Filtering Students' Mathematics Course Grades

db.subjectScores.find({
    $expr: { $eq: [ "$subject", "math" ] }
})

Equivalent to:

db.subjectScores.find({
    "subject": "math"
})

Equivalent to:

db.subjectScores.find({
    "subject": {
        $eq: "math"
    }
})

The result of the query is as follows:

{ "_id" : 4, "name" : "张三", "subject" : "math", "score" : 70 }
{ "_id" : 5, "name" : "李四", "subject" : "math", "score" : 90 }
{ "_id" : 6, "name" : "王五", "subject" : "math", "score" : 50 }

3. Use the js function to query the document ($where)

grammar:

{ $where: <JavaScript Code> }

Query documents using js functions

Example: Filtering Students' Mathematics Course Grades

db.subjectScores.find({
    $where: function() {
        return this.subject === "math";
    }
})

Alternatively, we can use the aggregate expression operator $function in $expr instead of $where :

db.subjectScores.find({
    $expr: {
        $function: {
            body: function(s) {
                return s === "math";
            },
            args: [ "$subject" ],
            lang: "js"
        }
    }
})

If you don't know much about $function , you can refer to:

The result of using the custom aggregate expression operator ($function) https://blog.csdn.net/m1729339749/article/details/130597563 query in the MongoDB aggregation pipeline is as follows:

{ "_id" : 4, "name" : "张三", "subject" : "math", "score" : 70 }
{ "_id" : 5, "name" : "李四", "subject" : "math", "score" : 90 }
{ "_id" : 6, "name" : "王五", "subject" : "math", "score" : 50 }