MongoDB: what is the difference between $elemMatch and $and to find objects inside array?

Question

Is there any logical difference between the usage of the query operator $and

db.collection.find({$and: [{\"array.field1\": \"someValue\"}, {\"array.         


        

Answer 1

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
      }
    ]
  }

The first query

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

will returne the three documents,

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

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

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