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Combines multiple documents into a single document.将多个文档合并为一个文档。
$mergeObjects is available in these stages:可在以下阶段使用：
When used as a 当用作$bucket, $bucketAuto, or $group stage accumulator, $mergeObjects has this syntax:$bucket、$bucketAuto或$group阶段累加器时，$mergeObjects具有以下语法：
{ $mergeObjects: <document> }
When used in other expressions (including in 在其他表达式（包括$bucket, $bucketAuto, and $group stages) but not as an accumulator, $mergeObjects has this syntax:$bucket、$bucketAuto和$group阶段）中使用但不作为累加器时，$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。
$mergeObjects ignores 忽略null operands. null操作数。If all the operands to 如果$mergeObjects resolves to null, $mergeObjects returns an empty document { }.$mergeObjects的所有操作数都解析为null，$mergeObjects将返回一个空文档{}。
$mergeObjects overwrites the field values as it merges the documents. 合并文档时覆盖字段值。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.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具有该字段最后一个合并文档中的值。
{ $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 }
|
$mergeObjectsCreate a collection 使用以下文档创建集合orders with the following documents:orders：
db.orders.insertMany( [
{ "_id" : 1, "item" : "abc", "price" : 12, "ordered" : 2 },
{ "_id" : 2, "item" : "jkl", "price" : 20, "ordered" : 1 }
] )
Create another collection 使用以下文档创建另一个集合items with the following documents:items：
db.items.insertMany( [
{ "_id" : 1, "item" : "abc", description: "product 1", "instock" : 120 },
{ "_id" : 2, "item" : "def", description: "product 2", "instock" : 80 },
{ "_id" : 3, "item" : "jkl", description: "product 3", "instock" : 60 }
] )
The following operation first uses the 以下操作首先使用$lookup stage to join the two collections by the item fields and then uses $mergeObjects in the $replaceRoot to merge the joined documents from items and orders:$lookup阶段通过item字段联接两个集合，然后使用$replaceRoot中的$mergeObjects合并来自项和订单的联接文档：
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
}
$mergeObjectsCreate a collection 使用以下文档创建集合sales with the following documents:sales：
db.sales.insertMany( [
{ _id: 1, year: 2017, item: "A", quantity: { "2017Q1": 500, "2017Q2": 500 } },
{ _id: 2, year: 2016, item: "A", quantity: { "2016Q1": 400, "2016Q2": 300, "2016Q3": 0, "2016Q4": 0 } } ,
{ _id: 3, year: 2017, item: "B", quantity: { "2017Q1": 300 } },
{ _id: 4, year: 2016, item: "B", quantity: { "2016Q3": 100, "2016Q4": 250 } }
] )
The following operation uses 以下操作将$mergeObjects as a accumulator in a $group stage that groups documents by the item field:$mergeObjects用作按项目字段对文档进行分组的$group阶段中的累加器：
When used as an accumulator, 当用作累加器时，$mergeObjects operator accepts a single operand.$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 }
}
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，则结果文档中的字段具有该字段最后一个合并文档中的值。