MongoDB query multiple collections at once

Question

users      id   12345     admin  1         id   123456789     admin  0    posts     content   Some content     owner id   12345     via   facebook         content   Some other content     owner id   123456789     via   facebook      Here is a sample from my mongodb   I want to get all the posts which has  via  attribute equal to  facebook  and posted by an admin   admin  1    I couldn t figure out how to acquire this query   Since mongodb is not a relational database  I couldn t do a join operation   What could be the solution

User · Answer

Here is answer for your question   db getCollection  users   aggregate         match    admin   1          lookup   from   posts  localField    id  foreignField   owner id  as   posts           project                 posts      filter    input     posts     as    post   cond      eq       post via     facebook                      admin   1                   Or either you can go with mongodb group option   db getCollection  users   aggregate         match    admin   1          lookup   from   posts  localField    id  foreignField   owner id  as   posts           unwind     posts          match     posts via   facebook            group                  id      id               posts     push     posts

User · Answer

As mentioned before in MongoDB you can t JOIN between collections   For your example a solution could be   var myCursor   db users find  admin 1    var user id   myCursor hasNext     myCursor next     null  db posts find  owner id   user id  id      See the reference manual - cursors section  http   es docs mongodb org manual core cursors   Other solution would be to embed users in posts collection  but I think for most web applications users collection need to be independent for security reasons  Users collection might have Roles  permissons  etc   posts     content   Some content     user     id   12345    admin  1     via   facebook         content   Some other content     user     id   123456789    admin  0     via   facebook      and then   db posts find  user admin  1

User · Answer

One solution  add isAdmin  0 1 flag to your post collection document   Other solution  use DBrefs

User · Answer

Perform multiple queries or use embedded documents or look at  database references

User · Answer

You can use  lookup   multiple   to get the records from multiple collections   Example   If you have more collections   I have 3 collections for demo here  you can have more than 3    and I want to get the data from 3 collections in single object   The collection are as   db doc1 find   pretty             id    ObjectId  5901a4c63541b7d5d3293766         firstName     shubham        lastName     verma      db doc2 find   pretty             id    ObjectId  5901a5f83541b7d5d3293768         userId    ObjectId  5901a4c63541b7d5d3293766         address     Gurgaon        mob     9876543211      db doc3 find   pretty             id    ObjectId  5901b0f6d318b072ceea44fb         userId    ObjectId  5901a4c63541b7d5d3293766         fbURLs     http   www facebook com        twitterURLs     http   www twitter com      Now your query will be as below   db doc1 aggregate          match     id  ObjectId  5901a4c63541b7d5d3293766                       lookup                        from   doc2               localField    id               foreignField   userId               as   address                                  unwind    address                        project                  v  0               address   v   0               address  id   0               address userId   0               address mob   0                                 lookup                        from   doc3               localField    id               foreignField   userId               as   social                                  unwind    social                      project                       v  0                    social   v   0                    social  id   0                    social userId   0                 pretty      Then Your result will be           id    ObjectId  5901a4c63541b7d5d3293766         firstName     shubham        lastName     verma         address               address     Gurgaon              social               fbURLs     http   www facebook com            twitterURLs     http   www twitter com            If you want all records from each collections then you should remove below line from query                  project                      v  0                   address   v   0                   address  id   0                   address userId   0                   address mob   0                                        project                          social   v   0                        social  id   0                        social userId   0                       After removing above code you will get total record as           id    ObjectId  5901a4c63541b7d5d3293766         firstName     shubham        lastName     verma        address                id    ObjectId  5901a5f83541b7d5d3293768             userId    ObjectId  5901a4c63541b7d5d3293766             address     Gurgaon            mob     9876543211              social                id    ObjectId  5901b0f6d318b072ceea44fb             userId    ObjectId  5901a4c63541b7d5d3293766             fbURLs     http   www facebook com            twitterURLs     http   www twitter com

User · Answer

Trying to JOIN in MongoDB would defeat the purpose of using MongoDB  You could  however  use a DBref and write your application-level code  or library  so that it automatically fetches these references for you   Or you could alter your schema and use embedded documents   Your final choice is to leave things exactly the way they are now and do two queries

[collections] MongoDB query multiple collections at once

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