Database Manual / Reference / Query Language / Accumulators

$mergeObjects (accumulator operator)（累加器运算符）

Definition定义

$mergeObjects

Combines multiple documents into a single document.将多个文档合并为一个文档。

$mergeObjects is available in these stages:在以下阶段可用：

Syntax语法

When used as a $bucket, $bucketAuto, or $group stage accumulator, $mergeObjects has this syntax:当用作$bucket、$bucketAuto或$group阶段累加器时，$mergeObjects具有以下语法：

{ $mergeObjects: <document> }

When used in other expressions (including in $bucket, $bucketAuto, and $group stages) but not as an accumulator, $mergeObjects has this syntax:当在其他表达式中使用（包括在$bucket、$bucketAuto和$group阶段）但不作为累加器时，$mergeObjects具有以下语法：

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

The <document> can be any valid expression that resolves to a document.<document>可以是解析为文档的任何有效表达式。

Behavior行为

$mergeObjects ignores null operands. If all the operands to $mergeObjects resolves to null, $mergeObjects returns an empty document { }.$mergeObjects忽略空操作数。如果$mergeObjects的所有操作数都解析为null，则$mergeObject将返回一个空文档{ }。

$mergeObjects overwrites the field values as it merges the documents. If documents to merge include the same field name, the field, in the resulting document, has the value from the last document merged for the field.$mergeObjects在合并文档时会覆盖字段值。如果要合并的文档包含相同的字段名，则结果文档中的字段将具有该字段上次合并文档的值。

Example示例Results结果
 
{ $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示例

$mergeObjects

Create a collection orders with the following documents:使用以下文档创建集合orders：

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:使用以下文档创建另一个集合items：

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 $replaceRoot to merge the joined documents from items and orders:以下操作首先使用$lookup阶段按item字段连接两个集合，然后使用$replaceRoot中的$mergeObjects合并items和orders中的连接文档：

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:该操作返回以下文档：

{
  _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作为累加器

Create a collection sales with the following documents:使用以下文档创建集合sales：

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 $group stage that groups documents by the item field:以下操作使用$mergeObjects作为$group阶段中的累加器，该阶段按item字段对文档进行分组：

Note

When used as an accumulator, $mergeObjects operator accepts a single operand.当用作累加器时，$mergeObjects运算符接受单个操作数。

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

The operation returns the following documents:该操作返回以下文档：

{
  _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.如果要合并的文档包含相同的字段名，则生成文档中的字段将具有该字段上次合并文档的值。