MongoDB and joins

Question

I m sure MongoDB doesn t officially support  joins   What does this mean   Does this mean  We cannot connect two collections tables  together     I think if we put the value for  id in collection A to the other id in collection B  can we simply connect two collections   If my understanding is correct  MongoDB can connect two tables together  say  when we run a query  This is done by  Reference  written in http   www mongodb org display DOCS Schema Design   Then what does  joins  really mean   I d love to know the answer because this is essential to learn MongoDB schema design  http   www mongodb org display DOCS Schema Design

User · Accepted Answer

It s no join since the relationship will only be evaluated when needed  A join  in a SQL database  on the other hand will resolve relationships and return them as if they were a single table  you  join two tables into one     You can read more about DBRef here  http   docs mongodb org manual applications database-references   There are two possible solutions for resolving references  One is to do it manually  as you have almost described  Just save a document s  id in another document s other id  then write your own function to resolve the relationship  The other solution is to use DBRefs as described on the manual page above  which will make MongoDB resolve the relationship client-side on demand  Which solution you choose does not matter so much because both methods will resolve the relationship client-side  note that a SQL database resolves joins on the server-side

User · Answer

If you use mongoose  you can just use assuming you re using subdocuments and population    Profile findById profileId    select  friends     exec  err  profile  - gt      if err or not profile       handleError err  profile  res     else       Status find   profile     in  profile friends       err  statuses  - gt          if err           handleErr err  statuses  res         else           res json createJSON statuses   It retrieves Statuses which belong to one of Profile  profileId  friends  Friends is array of references to other Profiles  Profile schema with friends defined   schema   new mongoose Schema            friends        type  mongoose Schema Types ObjectId     ref   Profile      unique  true     index  true

User · Answer

I came across lot of posts searching for the same -  Mongodb Joins  and alternatives or equivalents  So my answer would help many other who are like me  This is the answer I would be looking for   I am using Mongoose with Express framework  There is a functionality called Population in place of joins   As mentioned in Mongoose docs      There are no joins in MongoDB but sometimes we still want references to documents in other collections  This is where population comes in    This StackOverflow answer shows a simple example on how to use it

User · Answer

one kind of join a query in mongoDB  is ask at one collection for id that match   put ids in a list  idlist    and do find using on other  or same  collection with  in   idlist  u   db friends find   friends   something    toArray   idlist     u forEach function myDoc    idlist push myDoc id        db family find   id     in   idlist

User · Answer

The first example you link to shows how MongoDB references behave much like lazy loading not like a join   There isn t a query there that s happening on both collections  rather you query one and then you lookup items from another collection by reference

User · Answer

The database does not do joins -- or automatic  linking  between documents   However you can do it yourself client side   If you need to do 2  that is ok  but if you had to do 2000  the number of client server turnarounds would make the operation slow   In MongoDB a common pattern is embedding  In relational when normalizing things get broken into parts   Often in mongo these pieces end up being a single document  so no join is needed anyway   But when one is needed  one does it client-side   Consider the classic ORDER  ORDER-LINEITEM example   One order and 8 line items are 9 rows in relational  in MongoDB we would typically just model this as a single BSON document which is an order with an array of embedded line items   So in that case  the join issue does not arise   However the order would have a CUSTOMER which probably is a separate collection - the client could read the cust id from the order document  and then go fetch it as needed separately     There are some videos and slides for schema design talks on the mongodb org web site I belive

User · Answer

The fact that mongoDB is not relational have led some people to consider it useless   I think that you should know what you are doing before designing a DB  If you choose to use noSQL DB such as MongoDB  you better implement a schema  This will make your collections - more or less - resemble tables in SQL databases  Also  avoid denormalization  embedding   unless necessary for efficiency reasons    If you want to design your own noSQL database  I suggest to have a look on Firebase documentation  If you understand how they organize the data for their service  you can easily design a similar pattern for yours   As others pointed out  you will have to do the joins client-side  except with Meteor  a Javascript framework   you can do your joins server-side with this package  I don t know of other framework which enables you to do so   However  I suggest you read this article before deciding to go with this choice   Edit 28 04 17  Recently Firebase published this excellent series on designing noSql Databases  They also highlighted in one of the episodes the reasons to avoid joins and how to get around such scenarios by denormalizing your database

[mongodb] MongoDB and "joins"

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