MongoDB:$ elemMatch和$ and和在数组内查找对象之间有什么区别? [英] MongoDB: what is the difference between $elemMatch and $and to find objects inside array?

问题描述

查询运算符$and

db.collection.find({$and: [{"array.field1": "someValue"}, {"array.field2": 3}]})

和投影运算符$elemMatch

db.collection.find({array: {$elemMatch: {field1: "someValue", field2: 3}}})

查找包含数组中的对象字段的文档?

to find documents which contain the object fields inside an array?

推荐答案

您的第一个查询将找到文档，其中数组具有至少一个具有field1 = somevalue的元素和至少一个具有field2 = 3的元素.这两个元素可以不同. 第二个将检索文档，其中数组具有至少一个同时匹配两个条件的元素. 这是一个数据样本来说明:

Your first query will find documents, where array have at least one element with field1= somevalue and at least one element with field2=3. Both elements can be different. The second one will retrieve documents where array have at least one element matching the two conditions simultaneously. Here's a data sample to explain :

   {
    array: [
      {
        field1: 1,   
      },
      {
        field2: 2
      },
      {
        field1: 1,
        field2: 3
      },
    ]
  },
  {
    array: [
      {
        field1: 1,
        field2: 2
      },
      {
        field2: 3
      }
    ]
  },
  {
    array: [
      {
        field1: 1,
        field2: "other"
      },
      {
        field2: 2
      }
    ]
  }

第一个查询

db.collection.find({"array.field1":1，"array.field2":2})(等效 到您的$ and语法)

db.collection.find({"array.field1": 1,"array.field2": 2}) (equivalent to your $and syntax)

将返回三个文档，

db.collection.find({array:{$ elemMatch:{field1:1，field2:2}}})

db.collection.find({array: {$elemMatch: {field1: 1, field2: 2}}})

将仅返回第二个文档(唯一一个具有与两个条件都匹配的元素的文档)

will return only the second document (the only one having an element matching both criterias)

第一个查询的逻辑运算符是OR，第二个查询的逻辑运算符是AND，在数组元素级别.

EDIT : The logical operator of the first query is OR, for the second one it's AND, at level of array element.

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