有一个User的集合

{
    "_id" : ObjectId("5a0d45ca8af3a91847b7cf95"),
    "updatedAt" : ISODate("2017-11-16T09:34:14.651Z"),
    "createdAt" : ISODate("2017-11-16T08:01:14.119Z"),
    "name" : "John",
    "email" : "test1@gmail.com",
    "groupsFavorite" : [ 
        ObjectId("5a0d45db8af3a91847b7cf96")
    ],
    "groups" : [ 
        ObjectId("5a0d45db8af3a91847b7cf96"), 
        ObjectId("5a0d45e18af3a91847b7cf97")
    ],
    "__v" : 3
}

有一组集合

/* 1 */
{
    "_id" : ObjectId("5a0d45db8af3a91847b7cf96"),
    "updatedAt" : ISODate("2017-11-16T08:01:31.815Z"),
    "createdAt" : ISODate("2017-11-16T08:01:31.815Z"),
    "userId" : ObjectId("5a0d45ca8af3a91847b7cf95"),
    "title" : "New title",
    "slug" : "new-title-1",
    "description" : "Lorem",
    "__v" : 0
}

/* 2 */
{
    "_id" : ObjectId("5a0d45e18af3a91847b7cf97"),
    "updatedAt" : ISODate("2017-11-16T08:01:37.005Z"),
    "createdAt" : ISODate("2017-11-16T08:01:37.005Z"),
    "userId" : ObjectId("5a0d45ca8af3a91847b7cf95"),
    "title" : "New title",
    "slug" : "new-title-2",
    "description" : "Lorem",
    "__v" : 0
}

/* 3 */
{
    "_id" : ObjectId("5a0d5cb0cd59342da943d55a"),
    "updatedAt" : ISODate("2017-11-16T09:38:56.912Z"),
    "createdAt" : ISODate("2017-11-16T09:38:56.912Z"),
    "userId" : ObjectId("5a0d5c48cd59342da943d559"),
    "title" : "New title",
    "slug" : "new-title-3",
    "description" : "Lorem",
    "__v" : 0
}

有一种方法可以返回特定用户的组。 有必要将这些数据添加到favorite: true 或false 的新属性中以建表。

例如:

{
    "_id" :"5a0d45db8af3a91847b7cf96",
    "updatedAt" : "2017-11-16T08:01:31.815Z",
    "createdAt" : "2017-11-16T08:01:31.815Z",
    "userId" : "5a0d45ca8af3a91847b7cf95",
    "title" : "New title",
    "slug" : "new-title-1",
    "description" : "Lorem",
    "favorite": "true"
},
{
    "_id" : "5a0d45e18af3a91847b7cf97",
    "updatedAt" : "2017-11-16T08:01:37.005Z",
    "createdAt" : "2017-11-16T08:01:37.005Z",
    "userId" : "5a0d45ca8af3a91847b7cf95",
    "title" : "New title",
    "slug" : "new-title-2",
    "description" : "Lorem",
    "favorite": "false"
},
{
    "_id" : "5a0d5cb0cd59342da943d55a",
    "updatedAt" : "2017-11-16T09:38:56.912Z",
    "createdAt" : "2017-11-16T09:38:56.912Z",
    "userId" : "5a0d5c48cd59342da943d559",
    "title" : "New title",
    "slug" : "new-title-3",
    "description" : "Lorem",
    "favorite": "false"
}

最佳答案

您可以使用 $lookup 运算符来检查“用户”集合中的 _id 和 userId 字段。并且为了确定它是否存在，您可以使用 $eq 运算符。

db.getCollection('Groups').aggregate([
   {
      $lookup:
      {
        from: "User",
        localField: "_id",
        foreignField: "groupsFavorite",
        as: "FavoriteByGrp"
      }
   }
  ,{
      $lookup:
      {
        from: "User",
        localField: "userId",
        foreignField: "_id",
        as: "FavoriteByUsr"
      }
   }
  ,{
       "$project":
       {
           _id:1,
           updatedAt:1,
           createdAt:1,
           userId:1,
           title:1,
           slug:1,
           description:1,
           favorite: 
           { 
               "$cond": 
               { 
                   if: { "$eq": [ "$FavoriteByGrp._id", "$FavoriteByUsr._id" ] }, 
                   then: "true", 
                   else: "false" 
               } 
           }
       }

   }
 ])

结果:

/* 1 */
{
    "_id" : ObjectId("5a0d45db8af3a91847b7cf96"),
    "updatedAt" : ISODate("2017-11-16T08:01:31.815Z"),
    "createdAt" : ISODate("2017-11-16T08:01:31.815Z"),
    "userId" : ObjectId("5a0d45ca8af3a91847b7cf95"),
    "title" : "New title",
    "slug" : "new-title-1",
    "description" : "Lorem",
    "favorite" : "true"
}

/* 2 */
{
    "_id" : ObjectId("5a0d45e18af3a91847b7cf97"),
    "updatedAt" : ISODate("2017-11-16T08:01:37.005Z"),
    "createdAt" : ISODate("2017-11-16T08:01:37.005Z"),
    "userId" : ObjectId("5a0d45ca8af3a91847b7cf95"),
    "title" : "New title",
    "slug" : "new-title-2",
    "description" : "Lorem",
    "favorite" : "false"
}

/* 3 */
{
    "_id" : ObjectId("5a0d5cb0cd59342da943d55a"),
    "updatedAt" : ISODate("2017-11-16T09:38:56.912Z"),
    "createdAt" : ISODate("2017-11-16T09:38:56.912Z"),
    "userId" : ObjectId("5a0d5c48cd59342da943d559"),
    "title" : "New title",
    "slug" : "new-title-3",
    "description" : "Lorem",
    "favorite" : "false"
}

关于mongodb - 向输出 json 添加其他属性，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/47330806/