`$lookup`

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Syntax

from Field Object
Examples
Basic Example
Nested Example

The MongoDB server $lookup performs a left outer join of one unsharded collection to another unsharded collection in the same database. Lookups are useful as they allow you to filter in documents from the "joined" collection for processing.

In federated database instance, you can use $lookup to join sharded and unsharded collections from the same database or different databases from Atlas, AWS S3, and HTTP or HTTPS data stores.

Note

For sharded collections, $lookup is only available on Atlas clusters running MongoDB 5.1 and later.

Syntax

The $lookup syntax is described in the MongoDB server manual.

In Data Federation, the from field in $lookup has the following alternate syntax. This allows you to specify an object that contains an optional database name and a required collection name:

{
  $lookup: {
    localField: "<fieldName>",
    from: <collection-to-join>|{db: <db>, coll: <collection-to-join>},
    foreignField: "<fieldName>",
    as: "<output-array-field>",
  }
}

{
  $lookup: {
    from: <collection to join>|{db: <db>, coll: <collection-to-join>},
    let: { <var_1>: <expression>, …, <var_n>: <expression> },
    pipeline: [ <pipeline to execute on the collection to join> ],
    as: <output array field>
  }
}

`from` Field Object

Field	Type	Description	Necessity
`db`	string	The database name. If you specify a database name, Data Federation reads data from the collection in the specified database. If you specify a database name that differs from the database upon which the command is operating, all nested $lookup stages must also specify this database name. If you don't specify a database name within a $lookup stage, collections in the stage inherit the database name specified in the closest parent $lookup stage if it exists, or the name of the database upon which the command is operating.	Conditional
`coll`	string	The collection name.	Required

Examples

Suppose there are three databases named sourceDB1, sourceDB2, and sourceDB3 with the following collections:

db.orders.insertMany([
  { "_id" : 1, "item" : "almonds", "price" : 12, "quantity" : 2 },
  { "_id" : 2, "item" : "pecans", "price" : 20, "quantity" : 1 },
  { "_id" : 3  }
])

db.catalog.insertMany([
  { "_id" : 1, "sku" : "almonds", "description": "product 1" },
  { "_id" : 2, "sku" : "bread", "description": "product 2" },
  { "_id" : 3, "sku" : "cashews", "description": "product 3" },
  { "_id" : 4, "sku" : "pecans", "description": "product 4" },
  { "_id" : 5, "sku": null, "description": "Incomplete" },
  { "_id" : 6 }
])

db.warehouses.insertMany([
  { "_id" : 1, "stock_item" : "almonds", "warehouse": "A", "instock" : 120 },
  { "_id" : 2, "stock_item" : "pecans", "warehouse": "A", "instock" : 70 },
  { "_id" : 3, "stock_item" : "cashews", "warehouse": "B", "instock" : 60 },
  { "_id" : 4, "stock_item" : "bread", "warehouse": "B", "instock" : 80 },
  { "_id" : 5, "stock_item" : "cookies", "warehouse": "A", "instock" : 80 }
])

The following examples use the $lookup aggregation stage to join documents from one collection with the documents from the collection in the other databases.

Basic Example

db.getSiblingDb("sourceDB1").orders.aggregate(
  {
    $lookup: {
      from: { db: "sourceDB2", coll: "catalog" },
      localField: "item",
      foreignField: "sku",
      as: "inventory_docs"
    }
  }
)

Nested Example

The following aggregation operation on the sourceDB1.orders collection joins the documents from the orders collection with the documents from the sourceDB2.catalog collection and the documents from the sourceDB3.warehouses collection using the item field from the orders collection, the sku field from the catalog collection, and the stock_item and instock fields from the warehouses collection:

db.getSiblingDb("sourceDB1").orders.aggregate(
  [
    {
      $lookup: {
        from: db: "sourceDB2", coll: "catalog",
        let: { "order_sku": "$item" },
        pipeline: [
          {
            $match: {
              $expr: {
                $eq: ["$sku", "$$order_sku"]
              }
            }
          },
          {
            $lookup: {
              from: db: "sourceDB3", coll: "warehouses",
              pipeline: [
                {
                  $match: {
                    $expr:{
                      $eq : ["$stock_item", "$$order_sku"]
                    }
                  }
                },
                {
                  $project : { "instock": 1, "_id": 0}
                }
              ],
              as: "wh"
            }
          },
          { "$unwind": "$wh" },
          {
            $project : { "description": 1, "instock": "$wh.instock", "_id": 0}
          }
        ],
        as: "inventory"
      },
    },
  ]
)

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