$elemMatch

The $elemMatch operator is used to match documents that contain an array field with at least one element that matches all the specified query criteria. This operator is useful when you need to find array documents with specified element.

Syntax

db.collection.find({ <field>: { $elemMatch: { <query1>, <query2>, ... } } })

Parameters

Parameter Description
field The field in the document that contains the array to be queried.
query The conditions that at least one element in the array must satisfy.

Example

Let's understand the usage with sample json from stores dataset.

[
  {
    "_id": "91de5201-8194-44bf-848f-674e8df8bf5e",
    "name": "Adatum Corporation",
    "promotionEvents": [
      {
        "discounts": [
          { "categoryName": "DJ Cases", "discountPercentage": 6 },
          { "categoryName": "DJ Mixers", "discountPercentage": 14 }
        ]
      },
      {
        "discounts": [
          { "categoryName": "DJ Headphones", "discountPercentage": 19 },
          { "categoryName": "DJ Speakers", "discountPercentage": 13 }
        ]
      },
      {
        "discounts": [
          { "categoryName": "DJ Lighting", "discountPercentage": 12 },
          { "categoryName": "DJ Accessories", "discountPercentage": 6 }
        ]
      }
    ]
  }
]

Example 1: Find in an array for specific element among the list of elements

The example finds the first two documents in the stores collection that have at least one discount with the category name "DJ Lighting" in their promotionEvents array. The query only returns the _id and promotionEvents.discounts fields for those documents.

db.stores.find(  {"promotionEvents.discounts":{$elemMatch:{"categoryName":"DJ Lighting"}}}
                ,{ _id: 1, "promotionEvents.discounts": 1 }).limit(2)

We receive the following documents returned.

{
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "promotionEvents": [
      {
        "discounts": [
          {
            "categoryName": "DJ Turntables",
            "discountPercentage": 18
          },
          {
            "categoryName": "DJ Mixers",
            "discountPercentage": 15
          }
        ]
      },
      {
        "discounts": [
          {
            "categoryName": "DJ Lighting",
            "discountPercentage": 14
          },
          {
            "categoryName": "DJ Cases",
            "discountPercentage": 20
          }
        ]
      }
    ]
  },
  {
    "_id": "91de5201-8194-44bf-848f-674e8df8bf5e",
    "promotionEvents": [
      {
        "discounts": [
          {
            "categoryName": "DJ Cases",
            "discountPercentage": 6
          },
          {
            "categoryName": "DJ Mixers",
            "discountPercentage": 14
          }
        ]
      },
      {
        "discounts": [
          {
            "categoryName": "DJ Headphones",
            "discountPercentage": 19
          },
          {
            "categoryName": "DJ Speakers",
            "discountPercentage": 13
          }
        ]
      },
      {
        "discounts": [
          {
            "categoryName": "DJ Lighting",
            "discountPercentage": 12
          },
          {
            "categoryName": "DJ Accessories",
            "discountPercentage": 6
          }
        ]
      }
    ]
}

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