在一个视图中连接两个集合MongoDB；展开有什么作用

我有集合user和domain，我想要一个结合它们的视图.

db.createView( "userDomainRelation", "user", [
   {
      $lookup:
         {
            from: "domain",
            localField: "domain",
            foreignField: "_id",
            as: "matchDomain"
         }
   },
   {
      $project:
         {
           _id: 1,
           email: 1,
           firstName: 1,
           lastName: 1,
           accountType: 1,
           domain: 1,
           host: "$matchDomain.host",
           isOpenDomain: "$matchDomain.isOpenDomain",
           isPartnerDomain: "$matchDomain.isPartnerDomain"
         }
   }
] )

这是可行的，但我不希望domain的字段作为array.unwind:

db.createView( "userDomainRelation", "user", [
   {
      $lookup:
         {
            from: "domain",
            localField: "domain",
            foreignField: "_id",
            as: "matchDomain"
         }
   },
   {
      $project:
         {
           _id: 1,
           email: 1,
           firstName: 1,
           lastName: 1,
           accountType: 1,
           domain: 1,
           host: "$matchDomain.host",
           isOpenDomain: "$matchDomain.isOpenDomain",
           isPartnerDomain: "$matchDomain.isPartnerDomain"
         }
   },
   {
        $unwind: "$host"
    }
] )

对于每user，host总是存在于domain中.如果我也解开isOpenDomain和isPartnerDomain，我只得到isOpenDomain和isPartnerDomain出现在"域"中的记录.为什么会这样？我该如何解决这个问题？我想要所有的域，即使isOpenDomain或isPartnerDomain是不存在的.

db.createView( "userDomainRelation", "user", [
   {
      $lookup:
         {
            from: "domain",
            localField: "domain",
            foreignField: "_id",
            as: "matchDomain"
         }
   },
   {
      $project:
         {
           _id: 1,
           email: 1,
           firstName: 1,
           lastName: 1,
           accountType: 1,
           domain: 1,
           host: "$matchDomain.host",
           isOpenDomain: "$matchDomain.isOpenDomain",
           isPartnerDomain: "$matchDomain.isPartnerDomain"
         }
   },
   {
        $unwind: "$host"
    },
   {
        $unwind: "$isOpenDomain"
    },
   {
        $unwind: "$isPartnerDomain"
    }
] )