database design - Model a Q & A site in mongoDB -

i need model q & site stackoverflow in mongodb, here main issues.

i have 2 main objects - questions , users (just forget others answers etc...).

users ask questions. need display user question always. possible search questions , browse questions. safer user ask less 1000 questions.

users have reputation updated , current reputation should shown question time.

solution 1 : embed users inside questions -

no need perform join when users reputation changes, relevant questions should updated. , not easy display single users

solution 2 : model users , question separate collection (just in rdbms) -

now updating reputation not issue need join between users , question every time question retrieved. in rdbms. furthermore mongodb doesn't have joins , join 2 calls - 1 question , 1 user, if there 100 questions retrieve there'll 100 calls 100 separate users - not good.

soluion 3 : embed users inside questions , have separate collection users too

when updating, update user collection , embedded users, when displaying user - use users

so 1 should use ? or else best solved rdbms mysql ?. , how fast field updates in mongodb ?

i'd love use mongo because of speed , easiness route read requests replicas , sharding (if site grew out of single server unlikely anyway ;( )

solution 1 sonunds me incompleted because in way should have storage of users.

solution 2 can solution, , it's better rdbms because in document database can't make 'real' join , that's means can easy scale system.

in case if system not big so, choose solution #2.

solution 3 no need embedd within question user info, embed information need display.

so best solution high scalable system be:

users - primary store of user related info, in addition here can have questions count, reputation, answers count , statistic data need

questions {shortuserinfo {username, reputation, goldbadgetscount, ...} } - store questions user related info need display

answers particular question should embedded within question. , contais shortuserinfo(or so).

solution #3 give ability easy scale system , make super fast. when user update profile(or reputation) should update information within each question/answer, can work async, in case information can stale time, it's okay. can change profile , see user name can still old on questions/answers.

hope you.