在MongoDB中学习Map-Reduce(Learning Map-Reduce in MongoDB)

 在通过互联网搜索时，我发现可以通过map-reduce函数在mongodb中模拟连接。 浏览文档令人困惑。 
 
 我有两个集合：一个包含一个用户的朋友列表。 而另一个系列则是所有用户。 我想获取所有朋友的个人资料照片。 如何创建mongodb查询以获得所需的结果？ 
 
 USERS系列： 
 
{
    "_id" : ObjectId("524c194a6e3715ce0a000001"),
    "email" : "qwerty@abc.com",
    "password" : "",
    "phone" : "",
    "salt" : "",
    "upic" : "someuser2fd2751259bb7519d7b760ffee9b7fce203ad1f34.jpg",
    "username" : "someuser2"
}
{
    "_id" : ObjectId("524be475fafb35480a000001"),
    "email" : "",
    "password" : "",
    "phone" : "",
    "salt" : "",
    "upic" : "amitverma2522b7a52e054c350f78fd7f3558919f2e2dab58.jpg",
    "username" : "amitverma"
}
 
 每个用户集合的朋友： 
 
{
    "_id" : ObjectId("526547ed2389630000000001"),
    "friends" : [
        {
            "_id" : ObjectId("524be475fafb35480a000001"),
            "username" : "amitverma"
        },
            {
            "_id" : ObjectId("524be475fafb35480a000001"),
            "username" : "someuser2"
        }

    ],
    "upic" : "macbookfd2751259bb7519d7b760ffee9b7fce203ad1f34.jpg",
    "username" : "someuser"
}
 
 帮助将不胜感激。 

While searching through the internet, I found out that joins can be emulated in mongodb through the map-reduce function. Going through the docs was confusing. 
 
I have two collections: one with a list of friends of one user. And the other collection is of all the users. I want to fetch the profile pictures of all the friends. how do I create a mongodb query to get the desired results? 
 
The USERS collection:
 
{
    "_id" : ObjectId("524c194a6e3715ce0a000001"),
    "email" : "qwerty@abc.com",
    "password" : "",
    "phone" : "",
    "salt" : "",
    "upic" : "someuser2fd2751259bb7519d7b760ffee9b7fce203ad1f34.jpg",
    "username" : "someuser2"
}
{
    "_id" : ObjectId("524be475fafb35480a000001"),
    "email" : "",
    "password" : "",
    "phone" : "",
    "salt" : "",
    "upic" : "amitverma2522b7a52e054c350f78fd7f3558919f2e2dab58.jpg",
    "username" : "amitverma"
}
 
The friends of each user collection:
 
{
    "_id" : ObjectId("526547ed2389630000000001"),
    "friends" : [
        {
            "_id" : ObjectId("524be475fafb35480a000001"),
            "username" : "amitverma"
        },
            {
            "_id" : ObjectId("524be475fafb35480a000001"),
            "username" : "someuser2"
        }

    ],
    "upic" : "macbookfd2751259bb7519d7b760ffee9b7fce203ad1f34.jpg",
    "username" : "someuser"
}
 
Help would be appreciated. 

原文：https://stackoverflow.com/questions/20153024