The $first operator in MongoDB's aggregation framework is used to return the first element in an array or the first document within a group. It is particularly useful when you want to retrieve the earliest or first occurring element based on a specified ordering.

Syntax

{
  $group: {
    _id: <expression>,
    firstField: { $first: <expression> }
  }
}

• $group: The operator used to specify the grouping stage.
• _id: The expression to group by.
• firstField: The field to store the first element.
• $first: The operator to get the first element in the group.

Example Setup

First, let's insert some documents into the sales collection to work with:

db.sales.insertMany([
  { sale_id: 1, item: "apple", quantity: 10, date: new Date("2023-01-01") },
  { sale_id: 2, item: "banana", quantity: 5, date: new Date("2023-01-02") },
  { sale_id: 3, item: "orange", quantity: 8, date: new Date("2023-01-03") },
  { sale_id: 4, item: "apple", quantity: 15, date: new Date("2023-01-04") },
  { sale_id: 5, item: "banana", quantity: 7, date: new Date("2023-01-05") }
])

Example 1: Basic Usage of $first

Retrieve the first sale for each item.

Pipeline:

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

Explanation:

• $sort: Sorts the documents by the date field in ascending order.
• group: Groups the documents by the item field. The $first operator retrieves the first document in each group, which is the earliest sale due to the prior sorting.

This command results in documents like:

{
  "_id": "apple",
  "firstSale": { "sale_id": 1, "item": "apple", "quantity": 10, "date": "2023-01-01T00:00:00Z" }
}

Example 2: Using $first with a Different Field

Retrieve the first quantity sold for each item.

Pipeline:

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

Explanation:

• $sort: Sorts the documents by the date field in ascending order.
• group: Groups the documents by the item field. The $first operator retrieves the first quantity value in each group, which corresponds to the earliest sale due to the prior sorting.

This command results in documents like:

{
  "_id": "apple",
  "firstQuantity": 10
}

Example 3: Combining $first with Other Aggregation Operations

Retrieve the first sale and the total quantity sold for each item.

Pipeline:

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

Explanation:

• $sort: Sorts the documents by the date field in ascending order.
• group: Groups the documents by the item field. The $first operator retrieves the first document in each group, and the $sum operator calculates the total quantity sold for each item.

This command results in documents like:

{
  "_id": "apple",
  "firstSale": { "sale_id": 1, "item": "apple", "quantity": 10, "date": "2023-01-01T00:00:00Z" },
  "totalQuantity": 25
}

Use Cases

1. Data Retrieval: Retrieve the first occurrence of an event, such as the first sale of an item.
2. Reporting: Generate reports that require the earliest record, such as the first transaction of each day.
3. Data Processing: Use in conjunction with grouping to get the first record based on a specific order, such as the first log entry for each user.

The $first operator is a valuable tool in MongoDB's aggregation framework for retrieving the first element in a group. By understanding how to use the $first operator effectively, you can perform detailed data retrieval, generate reports, and process data in meaningful ways.