Node.js 聚合（管道）使用$unwind、$lookup、$group链接多个集合_Node.js_Mongodb_Mongoose_Aggregate_Lookup

Node.js 聚合（管道）使用$unwind、$lookup、$group链接多个集合

node.js mongodb mongoose

Node.js 聚合（管道）使用$unwind、$lookup、$group链接多个集合,node.js,mongodb,mongoose,aggregate,lookup,Node.js,Mongodb,Mongoose,Aggregate,Lookup,我不熟悉mongodb和mongoose的聚合特性，在通过我的管道传递数据后，一直很难获得所需的结果下面我使用一个虚构的示例模型简化了模型情景我有3个模型（Ship、Yatch和Sailboat），它们共享接口并从基类扩展。第四种型号，Captain，它有一个数组watercraftContext，其中包含用于引用与每个Captain关联的船艇类型的对象 Mongo数据/模式设置示例如您所见，对象数组已设置为使用mongoose.populate（）方法和ref和\u id字段，并且我已

我不熟悉

mongodb

和

mongoose

的聚合特性，在通过我的管道传递数据后，一直很难获得所需的结果

下面我使用一个虚构的示例模型简化了模型

情景我有3个模型（

Ship

、

Yatch

和

Sailboat

），它们共享接口并从基类扩展。第四种型号，

Captain

，它有一个数组

watercraftContext

，其中包含用于引用与每个

Captain

关联的

船艇类型的对象
Mongo数据/模式设置示例
如您所见，对象数组已设置为使用mongoose.populate（）
方法和ref
和\u id
字段，并且我已经实现了一个虚拟getter
船艇以使用populate（）
功能（代码未发布）
当使用mongoose.Model查询时，将创建一个新字段作为watercrafts
，其中包含来自3个不同关联集合的所有对象数组
问题
我还需要一种方法来对这些数据进行聚合
，以产生类似的结果，因为模型方法在聚合管线
中不可用
以下是从我的编程mongo聚合生成的查询：
[ { '$match':
     { _id:
        { '$in':
           [ ObjectId('5f77bc653887221a703415e1'),
             ObjectId('5f77bc653887221a703415df'),
             ObjectId('5f77bc653887221a703415e0'),
             ObjectId('5f77bc653887221a703415e5') ] } } },
  { '$unwind': '$watercraftContexts' },
  { '$lookup':
     { from: 'ships',
       localField: 'watercraftContexts._id',
       foreignField: '_id',
       as: 'watercrafts.ships' } },
  { '$unwind': '$watercraftContexts' },
  { '$lookup':
     { from: 'yatches',
       localField: 'watercraftContexts._id',
       foreignField: '_id',
       as: 'watercrafts.yatches' } },
  { '$unwind': '$watercraftContexts' },
  { '$lookup':
     { from: 'sailboats',
       localField: 'watercraftContexts._id',
       foreignField: '_id',
       as: 'watercrafts.sailboats' } },
  { '$group':
     { _id: '$_id',
       watercrafts:
        { '$addToSet':
           { '$concatArrays':
              [ '$watercrafts.ships',
                '$watercrafts.yatches',
                '$watercrafts.sailboats' ] } }

我正在构建一个猫鼬集合，如下所示：
const Captain = mongoose.model('Captain')
const aggregate = Captain.aggregrate()

// Dynamically create Aggregate Pipeline in another function
const captains = await Captain.find({})
const captainIds = captains.map(capt => capt._id)

// Match sub-set of documents (in actual project)
aggregate.match({ _id: { $in: captainIds } })

// Collection names to apply $lookup aggregate
const collectionNames = ['sailboats', 'yatches', 'ships']

// Unwind and Lookup for each polymorphic child class's collection
collectionNames.forEach(collection => {
  // Separate watercraftContexts into individual records for lookup
  aggregate.unwind('watercraftContexts')
  // Inner Join collection data on record
  aggregate.lookup({
    from: collection,
    localField: '$watercrafContexts._id',
    foreignField: '_id',
      // Object keyed by collection name with array of collection records
      // to avoid overwrite of previous collection aggregate lookup
    as: `watercrafts.${collection}`  
  })
})

// Re-group the records by Captain Object Id
const aggregateAssociationPaths = collectionNames.map(collection => 
  // Mongo Path to each collection $lookup
  `$watercrafts.${collection}`
)

// Re-assemble $unwind and $group by Captain's ObjectId
aggregate.group({
  _id: '$_id',
  $addToSet: {
    // 
    $concatArrays: aggregateAssociationPaths 
  }
})

/***  !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!  !!!  ***
 *                                                                                        *
 *    WHAT DO I DO NEXT TO GET ALL THE CAPTAIN DATA WITH THE AGGREGATED `watercrafts`
 *                                                                                        *
 ***  !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!   !!!  !!!  ***/

// Execute Aggregation
const captiansWithWatercraftsAssociations = await aggregate

到目前为止，我的数据是这样的，而且小组没有与猫鼬合作：
[ { _id: 5f77bc653887221a703415df,
    watercrafts:
     [ { _id: 5f77bc653887221a703415d3,
         class: 'sail',
         name: 'Gone with the Wind',
         __v: 0 } ] },
  { _id: 5f77bc653887221a703415e0,
    watercrafts:
     [ { _id: 5f77bc653887221a703415d4,
         class: 'yatch',
         name: 'Liquid Gold',
         __v: 0 } ] },
  { _id: 5f77bc653887221a703415e1,
    watercrafts:
     [ { _id: 5f77bc653887221a703415d5,
         class: 'ship',
         name: 'Jenny',
         __v: 0 } ] },
  { _id: 5f77bc653887221a703415e5,
    watercrafts:
     [ { _id: 5f77bc653887221a703415dd,
         class: 'yatch',
         name: 'Audrey',
         __v: 0 } ] },
  { _id: 5f77bc653887221a703415e5,
    watercrafts:
     [ { _id: 5f77bc653887221a703415dc,
         class: 'sail',
         name: 'Swell Shredder',
         __v: 0 } ] },
  { _id: 5f77bc653887221a703415e5,
    watercrafts:
     [ { _id: 5f77bc653887221a703415de,
         class: 'ship',
         name: 'Jenny IV',
         __v: 0 } ] } ]

感谢您的支持
对于刚接触MongoDb
的人来说，这是一个棘手的问题。我将把我的答案分解成几个步骤，向其他人演示如何使用引用多个集合来聚合数组
步骤1-$match在收集时筛选
$match is接受与db.collection.find（{}）
相同的查询，并返回匹配结果数组。在下面的例子中，我在这里选择了4条特定记录

{ '$match':
     { _id:
        { '$in':
           [
              ObjectId('5f7bdb3eea134b5a5c976285'),
              ObjectId('5f7bdb3eea134b5a5c976283'),
              ObjectId('5f7bdb3eea134b5a5c976284'),
              ObjectId('5f7bdb3eea134b5a5c976289')
           ]
        }
     }
}

$match结果
[ 
  { _id: ObjectId('5f7be0b37e2bdf5b19e4724d'),
    name: 'CAPTAIN_SAIL',
    classes: [ 'sail' ],
    license: 'WC-1',
    watercraftContexts:
     [ { _id: ObjectId('5f7be0b37e2bdf5b19e47241'),
         watercraftType: 'Sailboat',
         ref: 'sailboats' } ],
    __v: 0 },
  { _id: ObjectId('5f7be0b37e2bdf5b19e4724e'),
    name: 'CAPTAIN_YATCH',
    classes: [ 'yatch' ],
    license: 'WC-2',
    watercraftContexts:
     [ { _id: ObjectId('5f7be0b37e2bdf5b19e47242'),
         watercraftType: 'Yatch',
         ref: 'yatches' } ],
    __v: 0 },
  { _id: ObjectId('5f7be0b37e2bdf5b19e4724f'),
    name: 'CAPTAIN_SHIP',
    classes: [ 'ship' ],
    license: 'WC-3',
    watercraftContexts:
     [ { _id: ObjectId('5f7be0b37e2bdf5b19e47243'),
         watercraftType: 'Ship',
         ref: 'ships' } ],
    __v: 0 },
  { _id: ObjectId('5f7be0b37e2bdf5b19e47253'),
    name: 'CAPTAIN_SAIL_YATCH_SHIP',
    classes: [ 'sail', 'yatch', 'ship' ],
    license: 'WC-7',
    watercraftContexts:
     [ { _id: ObjectId('5f7be0b37e2bdf5b19e4724a'),
         watercraftType: 'Sailboat',
         ref: 'sailboats' },
       { _id: ObjectId('5f7be0b37e2bdf5b19e4724b'),
         watercraftType: 'Yatch',
         ref: 'yatches' },
       { _id: ObjectId('5f7be0b37e2bdf5b19e4724c'),
         watercraftType: 'Ship',
         ref: 'ships' } ],
    __v: 0 }
]

第2步-$REWIND，这样我们可以使用$loopup进行迭代
在这个结果集中有一个{u id:，watercraftType:}
的对象数组。要在数组上循环并将每个对象与各自的收集记录连接起来，我们必须将数组分解为单独的独立记录。$unwind
功能将为下一个聚合阶段创建新的数据集
  { '$unwind': '$watercraftContexts' },

$unwind结果
如您所见，$unwind
现在使用单个watercraftContext
创建记录，我们现在设置为使用$lookup

[ { _id: ObjectId('5f7be2231da37c5b5915bf9b'),
    name: 'CAPTAIN_SAIL',
    classes: [ 'sail' ],
    license: 'WC-1',
    watercraftContexts:
     { _id: ObjectId('5f7be2231da37c5b5915bf8f'),
       watercraftType: 'Sailboat',
       ref: 'sailboats' },
    __v: 0 },
  { _id: ObjectId('5f7be2231da37c5b5915bf9c'),
    name: 'CAPTAIN_YATCH',
    classes: [ 'yatch' ],
    license: 'WC-2',
    watercraftContexts:
     { _id: ObjectId('5f7be2231da37c5b5915bf90'),
       watercraftType: 'Yatch',
       ref: 'yatches' },
    __v: 0 },
  { _id: ObjectId('5f7be2231da37c5b5915bf9d'),
    name: 'CAPTAIN_SHIP',
    classes: [ 'ship' ],
    license: 'WC-3',
    watercraftContexts:
     { _id: ObjectId('5f7be2231da37c5b5915bf91'),
       watercraftType: 'Ship',
       ref: 'ships' },
    __v: 0 },
  { _id: ObjectId('5f7be2231da37c5b5915bfa1'),
    name: 'CAPTAIN_SAIL_YATCH_SHIP',
    classes: [ 'sail', 'yatch', 'ship' ],
    license: 'WC-7',
    watercraftContexts:
     { _id: ObjectId('5f7be2231da37c5b5915bf98'),
       watercraftType: 'Sailboat',
       ref: 'sailboats' },
    __v: 0 },
  { _id: ObjectId('5f7be2231da37c5b5915bfa1'),
    name: 'CAPTAIN_SAIL_YATCH_SHIP',
    classes: [ 'sail', 'yatch', 'ship' ],
    license: 'WC-7',
    watercraftContexts:
     { _id: ObjectId('5f7be2231da37c5b5915bf99'),
       watercraftType: 'Yatch',
       ref: 'yatches' },
    __v: 0 },
  { _id: ObjectId('5f7be2231da37c5b5915bfa1'),
    name: 'CAPTAIN_SAIL_YATCH_SHIP',
    classes: [ 'sail', 'yatch', 'ship' ],
    license: 'WC-7',
    watercraftContexts:
     { _id: ObjectId('5f7be2231da37c5b5915bf9a'),
       watercraftType: 'Ship',
       ref: 'ships' },
    __v: 0 } ]

步骤4$lookup-加入外部集合中的每个记录
需要注意的是，我们必须在为每个需要加入的不同集合调用$lookup
之前$unwind
。因为我们想要加入多个集合，所以我们需要将结果存储在由集合键入的对象中，以便以后聚合
  // Only performs $lookup on 'ships' collection
  { '$lookup':
     { from: 'ships',  // Collection Name - Note: repeat for each collection
       localField: 'watercraftContexts._id', // The field with id to link
       foreignField: '_id',  // The field on the foreign collection to match
       as: 'watercrafts.ships' // The path where to store the lookup result
     }
  }

步骤5-对其他联接重复$unwind和$lookup
对其他联接重复上述步骤，并按集合名称键入。我结合了聚合阶段来演示重复
  { '$unwind': '$watercraftContexts' },
  { '$lookup':
     { from: 'yatches',
       localField: 'watercraftContexts._id',
       foreignField: '_id',
       as: 'watercrafts.yatches' } },
  { '$unwind': '$watercraftContexts' },
  { '$lookup':
     { from: 'sailboats',
       localField: 'watercraftContexts._id',
       foreignField: '_id',
       as: 'watercrafts.sailboats' } }

步骤4和5结果
如果仔细观察，您会发现其中一条船长
记录以不同的船艇类型存在3次<代码>$lookup
将仅返回与特定集合名称匹配的记录。这就是为什么要将它们存储在由collectionName

[
  { _id: ObjectId('5f7be7145320a65b942bb450'),
    name: 'CAPTAIN_SAIL',
    classes: [ 'sail' ],
    license: 'WC-1',
    watercraftContexts:
     { _id: ObjectId('5f7be7145320a65b942bb444'),
       watercraftType: 'Sailboat',
       ref: 'sailboats' },
    __v: 0,
    watercrafts:
     { ships: [],
       yatches: [],
       sailboats:
        [ { _id: ObjectId('5f7be7145320a65b942bb444'),
            class: 'sail',
            name: 'Gone with the Wind',
            __v: 0 } ] } },
  { _id: ObjectId('5f7be7145320a65b942bb451'),
    name: 'CAPTAIN_YATCH',
    classes: [ 'yatch' ],
    license: 'WC-2',
    watercraftContexts:
     { _id: ObjectId('5f7be7145320a65b942bb445'),
       watercraftType: 'Yatch',
       ref: 'yatches' },
    __v: 0,
    watercrafts:
     { ships: [],
       yatches:
        [ { _id: ObjectId('5f7be7145320a65b942bb445'),
            class: 'yatch',
            name: 'Liquid Gold',
            __v: 0 } ],
       sailboats: [] } },
  { _id: ObjectId('5f7be7145320a65b942bb452'),
    name: 'CAPTAIN_SHIP',
    classes: [ 'ship' ],
    license: 'WC-3',
    watercraftContexts:
     { _id: ObjectId('5f7be7145320a65b942bb446'),
       watercraftType: 'Ship',
       ref: 'ships' },
    __v: 0,
    watercrafts:
     { ships:
        [ { _id: ObjectId('5f7be7145320a65b942bb446'),
            class: 'ship',
            name: 'Jenny',
            __v: 0 } ],
       yatches: [],
       sailboats: [] } },
  { _id: ObjectId('5f7be7145320a65b942bb456'),
    name: 'CAPTAIN_SAIL_YATCH_SHIP',
    classes: [ 'sail', 'yatch', 'ship' ],
    license: 'WC-7',
    watercraftContexts:
     { _id: ObjectId('5f7be7145320a65b942bb44d'),
       watercraftType: 'Sailboat',
       ref: 'sailboats' },
    __v: 0,
    watercrafts:
     { ships: [],
       yatches: [],
       sailboats:
        [ { _id: ObjectId('5f7be7145320a65b942bb44d'),
            class: 'sail',
            name: 'Swell Shredder',
            __v: 0 } ] } },
  { _id: ObjectId('5f7be7145320a65b942bb456'),
    name: 'CAPTAIN_SAIL_YATCH_SHIP',
    classes: [ 'sail', 'yatch', 'ship' ],
    license: 'WC-7',
    watercraftContexts:
     { _id: ObjectId('5f7be7145320a65b942bb44e'),
       watercraftType: 'Yatch',
       ref: 'yatches' },
    __v: 0,
    watercrafts:
     { ships: [],
       yatches:
        [ { _id: ObjectId('5f7be7145320a65b942bb44e'),
            class: 'yatch',
            name: 'Audrey',
            __v: 0 } ],
       sailboats: [] } },
  { _id: ObjectId('5f7be7145320a65b942bb456'),
    name: 'CAPTAIN_SAIL_YATCH_SHIP',
    classes: [ 'sail', 'yatch', 'ship' ],
    license: 'WC-7',
    watercraftContexts:
     { _id: ObjectId('5f7be7145320a65b942bb44f'),
       watercraftType: 'Ship',
       ref: 'ships' },
    __v: 0,
    watercrafts:
     { ships:
        [ { _id: ObjectId('5f7be7145320a65b942bb44f'),
            class: 'ship',
            name: 'Jenny IV',
            __v: 0 } ],
       yatches: [],
       sailboats: [] } } ]

步骤6$project-使用project展平连接的对象贴图
我们可以使用project选择所有现有数据，并将连接结果的对象映射展平到单个数组中
  { '$project':
     // keys with the value 'true' will be included
     { name: true,
       license: true,
       classes: true,
       _id: true,
       watercraftContexts: true,
       __v: true,
       watercrafts:            // Re-assigns value of watercrafts
        { '$setUnion':         // Accepts an array of arrays to flatten
           [
             '$watercrafts.ships',
             '$watercrafts.yatches',
             '$watercrafts.sailboats'
           ]
        }
     }
  }

$项目结果
上述$project
的结果将用船艇
的扁平数组替换船艇
对象，但需要注意的是，在匹配许多不同查找的情况下，船长
仍然存在重复记录。我们将在下一步中重新组装它们
[ { _id: ObjectId('5f7bea8d79dfe25bf3cb9695'),
    name: 'CAPTAIN_SAIL',
    classes: [ 'sail' ],
    license: 'WC-1',
    watercraftContexts:
     { _id: ObjectId('5f7bea8d79dfe25bf3cb9689'),
       watercraftType: 'Sailboat',
       ref: 'sailboats' },
    __v: 0,
    watercrafts:
     [ { _id: ObjectId('5f7bea8d79dfe25bf3cb9689'),
         class: 'sail',
         name: 'Gone with the Wind',
         __v: 0 } ] },
  { _id: ObjectId('5f7bea8d79dfe25bf3cb9696'),
    name: 'CAPTAIN_YATCH',
    classes: [ 'yatch' ],
    license: 'WC-2',
    watercraftContexts:
     { _id: ObjectId('5f7bea8d79dfe25bf3cb968a'),
       watercraftType: 'Yatch',
       ref: 'yatches' },
    __v: 0,
    watercrafts:
     [ { _id: ObjectId('5f7bea8d79dfe25bf3cb968a'),
         class: 'yatch',
         name: 'Liquid Gold',
         __v: 0 } ] },
  { _id: ObjectId('5f7bea8d79dfe25bf3cb9697'),
    name: 'CAPTAIN_SHIP',
    classes: [ 'ship' ],
    license: 'WC-3',
    watercraftContexts:
     { _id: ObjectId('5f7bea8d79dfe25bf3cb968b'),
       watercraftType: 'Ship',
       ref: 'ships' },
    __v: 0,
    watercrafts:
     [ { _id: ObjectId('5f7bea8d79dfe25bf3cb968b'),
         class: 'ship',
         name: 'Jenny',
         __v: 0 } ] },
  { _id: ObjectId('5f7bea8d79dfe25bf3cb969b'),
    name: 'CAPTAIN_SAIL_YATCH_SHIP',
    classes: [ 'sail', 'yatch', 'ship' ],
    license: 'WC-7',
    watercraftContexts:
     { _id: ObjectId('5f7bea8d79dfe25bf3cb9692'),
       watercraftType: 'Sailboat',
       ref: 'sailboats' },
    __v: 0,
    watercrafts:
     [ { _id: ObjectId('5f7bea8d79dfe25bf3cb9692'),
         class: 'sail',
         name: 'Swell Shredder',
         __v: 0 } ] },
  { _id: ObjectId('5f7bea8d79dfe25bf3cb969b'),
    name: 'CAPTAIN_SAIL_YATCH_SHIP',
    classes: [ 'sail', 'yatch', 'ship' ],
    license: 'WC-7',
    watercraftContexts:
     { _id: ObjectId('5f7bea8d79dfe25bf3cb9693'),
       watercraftType: 'Yatch',
       ref: 'yatches' },
    __v: 0,
    watercrafts:
     [ { _id: ObjectId('5f7bea8d79dfe25bf3cb9693'),
         class: 'yatch',
         name: 'Audrey',
         __v: 0 } ] },
  { _id: ObjectId('5f7bea8d79dfe25bf3cb969b'),
    name: 'CAPTAIN_SAIL_YATCH_SHIP',
    classes: [ 'sail', 'yatch', 'ship' ],
    license: 'WC-7',
    watercraftContexts:
     { _id: ObjectId('5f7bea8d79dfe25bf3cb9694'),
       watercraftType: 'Ship',
       ref: 'ships' },
    __v: 0,
    watercrafts:
     [ { _id: ObjectId('5f7bea8d79dfe25bf3cb9694'),
         class: 'ship',
         name: 'Jenny IV',
         __v: 0 } ] } ]

第7步$unwind和$group
我们$unwind
，以便我们现在可以将属于同一船长的所有船只进行分组。我们还必须使用$mergeObjects
将Captain
集合中的附加数据临时存储在一个新的临时变量下，为最终阶段做好准备
  { '$unwind': '$watercrafts' },
  { '$group':
     { _id: '$_id',
       data:
        { '$mergeObjects':
           { name: '$name',
             license: '$license',
             classes: '$classes',
             watercraftContexts: '$watercraftContexts',
             __v: '$__v' } },
       watercrafts: { '$push': '$watercrafts' } } }

$unwind
和$group结果
现在我们真的有进展了。我们已将转换减少到最初的4个Captain
s，并将连接平坦化为单个阵列
[ { _id: ObjectId('5f7bed5e271dd95c306c25a4'),
    data:
     { name: 'CAPTAIN_SHIP',
       license: 'WC-3',
       classes: [ 'ship' ],
       watercraftContexts:
        { _id: ObjectId('5f7bed5e271dd95c306c2598'),
          watercraftType: 'Ship',
          ref: 'ships' },
       __v: 0 },
    watercrafts:
     [ { _id: ObjectId('5f7bed5e271dd95c306c2598'),
         class: 'ship',
         name: 'Jenny',
         __v: 0 } ] },
  { _id: ObjectId('5f7bed5e271dd95c306c25a8'),
    data:
     { name: 'CAPTAIN_SAIL_YATCH_SHIP',
       license: 'WC-7',
       classes: [ 'sail', 'yatch', 'ship' ],
       watercraftContexts:
        { _id: ObjectId('5f7bed5e271dd95c306c25a1'),
          watercraftType: 'Ship',
          ref: 'ships' },
       __v: 0 },
    watercrafts:
     [ { _id: ObjectId('5f7bed5e271dd95c306c259f'),
         class: 'sail',
         name: 'Swell Shredder',
         __v: 0 },
       { _id: ObjectId('5f7bed5e271dd95c306c25a0'),
         class: 'yatch',
         name: 'Audrey',
         __v: 0 },
       { _id: ObjectId('5f7bed5e271dd95c306c25a1'),
         class: 'ship',
         name: 'Jenny IV',
         __v: 0 } ] },
  { _id: ObjectId('5f7bed5e271dd95c306c25a2'),
    data:
     { name: 'CAPTAIN_SAIL',
       license: 'WC-1',
       classes: [ 'sail' ],
       watercraftContexts:
        { _id: Object('5f7bed5e271dd95c306c2596'),
          watercraftType: 'Sailboat',
          ref: 'sailboats' },
       __v: 0 },
    watercrafts:
     [ { _id: ObjectId('5f7bed5e271dd95c306c2596'),
         class: 'sail',
         name: 'Gone with the Wind',
         __v: 0 } ] },
  { _id: ObjectId('5f7bed5e271dd95c306c25a3'),
    data:
     { name: 'CAPTAIN_YATCH',
       license: 'WC-2',
       classes: [ 'yatch' ],
       watercraftContexts:
        { _id: ObjectId('5f7bed5e271dd95c306c2597'),
          watercraftType: 'Yatch',
          ref: 'yatches' },
       __v: 0 },
    watercrafts:
     [ { _id: ObjectId('5f7bed5e271dd95c306c2597'),
         class: 'yatch',
         name: 'Liquid Gold',
         __v: 0 } ] } ]

步骤8$replaceRoot和$project
我们只剩下将数据
合并到每个记录的根中，并删除临时变量数据

  // Merges 'data' into the root of each record
  { '$replaceRoot': { newRoot: { '$mergeObjects': [ '$data', '$$ROOT' ] } } },
  // Use $project to remove data (include only the fields we want)
  { '$project':
     { name: true,
       license: true,
       classes: true,
       _id: true,
       watercraftContexts: true,
       __v: true,
       watercrafts: true } 
  }

$replaceRoot&$project Result
现在，我们得到了我们为……一名船长
和一系列混合相关类型的船艇

[ 
  { name: 'CAPTAIN_SAIL_YATCH_SHIP',
    license: 'WC-7',
    classes: [ 'sail', 'yatch', 'ship' ],
    watercraftContexts:
     { _id: ObjectId('5f7bf3b3680b375ca1755ea6'),
       watercraftType: 'Ship',
       ref: 'ships' },
    __v: 0,
    _id: ObjectId('5f7bf3b3680b375ca1755ead'),
    watercrafts:
     [ { _id: ObjectId('5f7bf3b3680b375ca1755ea4'),
         class: 'sail',
         name: 'Swell Shredder',
         __v: 0 },
       { _id: ObjectId('5f7bf3b3680b375ca1755ea5'),
         class: 'yatch',
         name: 'Audrey',
         __v: 0 },
       { _id: ObjectId('5f7bf3b3680b375ca1755ea6'),
         class: 'ship',
         name: 'Jenny IV',
         __v: 0 } ] },
  { name: 'CAPTAIN_SAIL',
    license: 'WC-1',
    classes: [ 'sail' ],
    watercraftContexts:
     { _id: ObjectId('5f7bf3b3680b375ca1755e9b'),
       watercraftType: 'Sailboat',
       ref: 'sailboats' },
    __v: 0,
    _id: ObjectId('5f7bf3b3680b375ca1755ea7'),
    watercrafts:
     [ { _id: ObjectId('5f7bf3b3680b375ca1755e9b'),
         class: 'sail',
         name: 'Gone with the Wind',
         __v: 0 } ] },
  { name: 'CAPTAIN_YATCH',
    license: 'WC-2',
    classes: [ 'yatch' ],
    watercraftContexts:
     { _id: ObjectId('5f7bf3b3680b375ca1755e9c'),
       watercraftType: 'Yatch',
       ref: 'yatches' },
    __v: 0,
    _id: ObjectId('5f7bf3b3680b375ca1755ea8'),
    watercrafts:
     [ { _id: ObjectId('5f7bf3b3680b375ca1755e9c'),
         class: 'yatch',
         name: 'Liquid Gold',
         __v: 0 } ] },
  { name: 'CAPTAIN_SHIP',
    license: 'WC-3',
    classes: [ 'ship' ],
    watercraftContexts:
     { _id: ObjectId('5f7bf3b3680b375ca1755e9d'),
       watercraftType: 'Ship',
       ref: 'ships' },
    __v: 0,
    _id: ObjectId('5f7bf3b3680b375ca1755ea9'),
    watercrafts:
     [ { _id: ObjectId('5f7bf3b3680b375ca1755e9d'),
         class: 'ship',
         name: 'Jenny',
         __v: 0 } ] } ]

现在你有了它…只花了2天就搞定了。如果您正在尝试类似的聚合关联，我希望它能为您节省一些时间。快乐编码
最终管道
如果您的问题是关于聚合管道的，您应该询问，使用可以在mongo shell中输入的代码。@D.SM添加了查询生成的聚合
[ 
  { name: 'CAPTAIN_SAIL_YATCH_SHIP',
    license: 'WC-7',
    classes: [ 'sail', 'yatch', 'ship' ],
    watercraftContexts:
     { _id: ObjectId('5f7bf3b3680b375ca1755ea6'),
       watercraftType: 'Ship',
       ref: 'ships' },
    __v: 0,
    _id: ObjectId('5f7bf3b3680b375ca1755ead'),
    watercrafts:
     [ { _id: ObjectId('5f7bf3b3680b375ca1755ea4'),
         class: 'sail',
         name: 'Swell Shredder',
         __v: 0 },
       { _id: ObjectId('5f7bf3b3680b375ca1755ea5'),
         class: 'yatch',
         name: 'Audrey',
         __v: 0 },
       { _id: ObjectId('5f7bf3b3680b375ca1755ea6'),
         class: 'ship',
         name: 'Jenny IV',
         __v: 0 } ] },
  { name: 'CAPTAIN_SAIL',
    license: 'WC-1',
    classes: [ 'sail' ],
    watercraftContexts:
     { _id: ObjectId('5f7bf3b3680b375ca1755e9b'),
       watercraftType: 'Sailboat',
       ref: 'sailboats' },
    __v: 0,
    _id: ObjectId('5f7bf3b3680b375ca1755ea7'),
    watercrafts:
     [ { _id: ObjectId('5f7bf3b3680b375ca1755e9b'),
         class: 'sail',
         name: 'Gone with the Wind',
         __v: 0 } ] },
  { name: 'CAPTAIN_YATCH',
    license: 'WC-2',
    classes: [ 'yatch' ],
    watercraftContexts:
     { _id: ObjectId('5f7bf3b3680b375ca1755e9c'),
       watercraftType: 'Yatch',
       ref: 'yatches' },
    __v: 0,
    _id: ObjectId('5f7bf3b3680b375ca1755ea8'),
    watercrafts:
     [ { _id: ObjectId('5f7bf3b3680b375ca1755e9c'),
         class: 'yatch',
         name: 'Liquid Gold',
         __v: 0 } ] },
  { name: 'CAPTAIN_SHIP',
    license: 'WC-3',
    classes: [ 'ship' ],
    watercraftContexts:
     { _id: ObjectId('5f7bf3b3680b375ca1755e9d'),
       watercraftType: 'Ship',
       ref: 'ships' },
    __v: 0,
    _id: ObjectId('5f7bf3b3680b375ca1755ea9'),
    watercrafts:
     [ { _id: ObjectId('5f7bf3b3680b375ca1755e9d'),
         class: 'ship',
         name: 'Jenny',
         __v: 0 } ] } ]

[ 
  { '$match':
     { _id:
        { '$in':
           [ ObjectId('5f7bf3b3680b375ca1755ea9'),
             ObjectId('5f7bf3b3680b375ca1755ea7'),
             ObjectId('5f7bf3b3680b375ca1755ea8'),
             ObjectId('5f7bf3b3680b375ca1755ead')
           ]
        }
     }
  },
  { '$unwind': '$watercraftContexts' },
  { '$lookup':
     { from: 'ships',
       localField: 'watercraftContexts._id',
       foreignField: '_id',
       as: 'watercrafts.ships' } },
  { '$unwind': '$watercraftContexts' },
  { '$lookup':
     { from: 'yatches',
       localField: 'watercraftContexts._id',
       foreignField: '_id',
       as: 'watercrafts.yatches' } },
  { '$unwind': '$watercraftContexts' },
  { '$lookup':
     { from: 'sailboats',
       localField: 'watercraftContexts._id',
       foreignField: '_id',
       as: 'watercrafts.sailboats' } },
  { '$project':
     { name: true,
       license: true,
       classes: true,
       _id: true,
       watercraftContexts: true,
       __v: true,
       watercrafts:
        { '$setUnion':
           [ '$watercrafts.ships',
             '$watercrafts.yatches',
             '$watercrafts.sailboats' ] } } },
  { '$unwind': '$watercrafts' },
  { '$group':
     { _id: '$_id',
       data:
        { '$mergeObjects':
           { name: '$name',
             license: '$license',
             classes: '$classes',
             watercraftContexts: '$watercraftContexts',
             __v: '$__v' } },
       watercrafts: { '$push': '$watercrafts' } } },
  { '$replaceRoot': { newRoot: { '$mergeObjects': [ '$data', '$$ROOT' ] } } },
  { '$project':
     { name: true,
       license: true,
       classes: true,
       _id: true,
       watercraftContexts: true,
       __v: true,
       watercrafts: true } }
]