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• $first (aggregation)

$first (aggregation)

Definition

$first

Returns the value that results from applying an expression to the first document in a group of documents that share the same group by key. Only meaningful when documents are in a defined order.

$first is only available in the $group stage.

$first has the following syntax:

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{ $first: <expression> }

For more information on expressions, see Expressions.

Behavior

When using $first in a $group stage, the output value depends on the order of the documents coming into pipeline. To guarantee a defined order, the $group pipeline stage should follow a $sort stage.

Missing Values

The documents in a group may be missing fields or may have fields with missing values.

• If there are no documents from the prior pipeline stage, the $group stage returns nothing.
• If the field that the $first accumulator is processing is missing, $first returns null.

See the missing data example.

Examples

Use in $group Stage

Create the sales collection:

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db.sales.insertMany( [
   { "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") },
   { "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") },
   { "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") },
   { "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") },
   { "_id" : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:05:00Z") },
   { "_id" : 6, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-15T12:05:10Z") },
   { "_id" : 7, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T14:12:12Z") }
] )

Grouping the documents by the item field, the following operation uses the $first accumulator to compute the first sales date for each item:

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db.sales.aggregate(
   [
     { $sort: { item: 1, date: 1 } },
     {
       $group:
         {
           _id: "$item",
           firstSale: { $first: "$date" }
         }
     }
   ]
)

The operation returns the following results:

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[
   { _id: 'jkl', firstSale: ISODate("2014-02-03T09:00:00.000Z") },
   { _id: 'xyz', firstSale: ISODate("2014-02-03T09:05:00.000Z") },
   { _id: 'abc', firstSale: ISODate("2014-01-01T08:00:00.000Z") }
]

Missing Data

Some documents in the badData collection are missing fields, other documents are missing values.

Create the badData collection:

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db.badData.insertMany( [
   { "_id": 1, "price": 6, "quantity": 6 },
   { "_id": 2, "item": "album", "price": 5 , "quantity": 5  },
   { "_id": 7, "item": "tape", "price": 6, "quantity": 6 },
   { "_id": 8, "price": 5, "quantity": 5  },
   { "_id": 9, "item": "album", "price": 3, "quantity": '' },
   { "_id": 10, "item": "tape", "price": 3, "quantity":  4 },
   { "_id": 12, "item": "cd", "price": 7  }
] )

Query the badData collection, grouping the output on the item field:

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db.badData.aggregate( [
   { $sort: { item: 1, price: 1 } },
   { $group:
      {
         _id: "$item",
         inStock: { $first: "$quantity" }
      }
   }
] )

The $sort stage orders the documents and passes them to the $group stage.

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[
  { _id: null, inStock: 5 },
  { _id: 'album', inStock: '' },
  { _id: 'cd', inStock: null },
  { _id: 'tape', inStock: 4 }
]

$first selects the first document from each output group:

• The _id: null group is included.
• When the accumulator field, $quantity in this example, is missing, $first returns null.

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