mongodb不支持关系数据库的join(Connection) Query

eg:

db.users.insert({name:'tmac',blogid:1});
db.blog.insert({id:1,detail:'tmacs blog'});

users has an attribute blogid and the id of blogs are one-to-one:

db.users.find().forEach(function(x){
var blogs_record = db.blogs.findOne({id:x.blogid});
if(blogs_record != null){
db.temp.insert({name:x.name,detail:blogs_record.detail});
}
)

eg:

db.users.insert({name:'tmac',blogid:1});
db.blog.insert({id:1,detail:'tmacs blog1'});
db.blog.insert({id:1,detail:'tmacs blog2'});

users has an attribute blogid and the id of blogs is a pair of n:

db.users.find().forEach(function(x){
db.blogs.findOne({id:x.blogid}).forEach(function(y){
db.temp.insert({name:x.name,y.detail});
})
)

Blogs can directly embed the author's information as a sub-document of blogs.

mongoose provides the populate method to implement join. But I don’t understand it either~
http://www.nodeclass.com/api/mongoose.html#guide_populate

A relatively simple but clumsy method is to check one table first, then forEach the result and then query another table like the answer above.