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Operators

$mergeObjects (aggregation)

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  • Definition
  • Syntax
  • Behavior
  • Examples

Definition

$mergeObjects

Combines multiple documents into a single document.

$mergeObjects is available in these stages:

  • $bucket

  • $bucketAuto

  • $group

  • $replaceRoot

Syntax

When used as a $bucket, $bucketAuto, or $group stage accumulator, $mergeObjects has this syntax:

{ $mergeObjects: <document> }

When used in other expressions (including in $bucket, $bucketAuto, and $group stages) but not as an accumulator, $mergeObjects has this syntax:

{ $mergeObjects: [ <document1>, <document2>, ... ] }

The <document> can be any valid expression that resolves to a document.

Behavior

$mergeObjects ignores null operands. If all the operands to $mergeObjects resolves to null, $mergeObjects returns an empty document { }.

$mergeObjects overwrites the field values as it merges the documents. If documents to merge include the same field name, the field, in the resulting document, has the value from the last document merged for the field.

Example
Results
{ $mergeObjects: [ { a: 1 }, null ] }
{ a: 1 }
{ $mergeObjects: [ null, null ] }
{ }
{
   $mergeObjects: [
      { a: 1 },
      { a: 2, b: 2 },
      { a: 3, c: 3 }
   ]
}
{ a: 3, b: 2, c: 3 }
{
  $mergeObjects: [
    { a: 1 },
    { a: 2, b: 2 },
    { a: 3, b: null, c: 3 }
  ]
}
{ a: 3, b: null, c: 3 }

Examples

$mergeObjects

Create a collection orders with the following documents:

db.orders.insertMany( [
  { "_id" : 1, "item" : "abc", "price" : 12, "ordered" : 2 },
  { "_id" : 2, "item" : "jkl", "price" : 20, "ordered" : 1 }
] )

Create another collection items with the following documents:

db.items.insertMany( [
  { "_id" : 1, "item" : "abc", description: "product 1", "instock" : 120 },
  { "_id" : 2, "item" : "def", description: "product 2", "instock" : 80 },
  { "_id" : 3, "item" : "jkl", description: "product 3", "instock" : 60 }
] )

The following operation first uses the $lookup stage to join the two collections by the item fields and then uses $mergeObjects in the $replaceRoot to merge the joined documents from items and orders:

db.orders.aggregate( [
   {
      $lookup: {
         from: "items",
         localField: "item",    // field in the orders collection
         foreignField: "item",  // field in the items collection
         as: "fromItems"
      }
   },
   {
      $replaceRoot: { newRoot: { $mergeObjects: [ { $arrayElemAt: [ "$fromItems", 0 ] }, "$$ROOT" ] } }
   },
   { $project: { fromItems: 0 } }
] )

The operation returns the following documents:

{
  _id: 1,
  item: 'abc',
  description: 'product 1',
  instock: 120,
  price: 12,
  ordered: 2
},
{
  _id: 2,
  item: 'jkl',
  description: 'product 3',
  instock: 60,
  price: 20,
  ordered: 1
}

$mergeObjects as an Accumulator

Create a collection sales with the following documents:

db.sales.insertMany( [
   { _id: 1, year: 2017, item: "A", quantity: { "2017Q1": 500, "2017Q2": 500 } },
   { _id: 2, year: 2016, item: "A", quantity: { "2016Q1": 400, "2016Q2": 300, "2016Q3": 0, "2016Q4": 0 } } ,
   { _id: 3, year: 2017, item: "B", quantity: { "2017Q1": 300 } },
   { _id: 4, year: 2016, item: "B", quantity: { "2016Q3": 100, "2016Q4": 250 } }
] )

The following operation uses $mergeObjects as a accumulator in a $group stage that groups documents by the item field:

Note

When used as an accumulator, $mergeObjects operator accepts a single operand.

db.sales.aggregate( [
   { $group: { _id: "$item", mergedSales: { $mergeObjects: "$quantity" } } }
] )

The operation returns the following documents:

{
  _id: 'A',
  mergedSales: { '2017Q1': 500, '2017Q2': 500, '2016Q1': 400, '2016Q2': 300, '2016Q3': 0, '2016Q4': 0 }
},
{
  _id: 'B',
  mergedSales: { '2017Q1': 300, '2016Q3': 100, '2016Q4': 250 }
}

Note

If the documents to merge include the same field name, the field in the resulting document has the value from the last document merged for the field.

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