MongoDB How to find out if an array field contains an element

Question

I have two collections   The first collection contains students       id    ObjectId  51780f796ec4051a536015cf     name     John        id    ObjectId  51780f796ec4051a536015d0     name     Sam        id    ObjectId  51780f796ec4051a536015d1     name     Chris        id    ObjectId  51780f796ec4051a536015d2     name     Joe      The second collection contains courses               id    ObjectId  51780fb5c9c41825e3e21fc4             name     CS 101            students                      ObjectId  51780f796ec4051a536015cf                    ObjectId  51780f796ec4051a536015d0                    ObjectId  51780f796ec4051a536015d2                           id    ObjectId  51780fb5c9c41825e3e21fc5             name     Literature            students                      ObjectId  51780f796ec4051a536015d0                    ObjectId  51780f796ec4051a536015d0                    ObjectId  51780f796ec4051a536015d2                           id    ObjectId  51780fb5c9c41825e3e21fc6             name     Physics            students                      ObjectId  51780f796ec4051a536015cf                    ObjectId  51780f796ec4051a536015d0                 Each course document contains students array which has a list of students registered for the course   When a student views a course on a web page he needs to see if he has already registered for the course or not   In order to do that  when the courses collection gets queried on the student s behalf  we need to find out if students array already contains the student s ObjectId   Is there a way to specify in the projection of a find query to retrieve student ObjectId from students array only if it is there   I tried to see if I could  elemMatch operator but it is geared towards an array of sub-documents   I understand that I could use aggregation framework but it seems that it would be on overkill in this case   Aggregation framework would probably not be as fast as a single find query   Is there a way to query course collection to so that the returned document could be in a form similar to this               id    ObjectId  51780fb5c9c41825e3e21fc4             name     CS 101            students                      ObjectId  51780f796ec4051a536015d0

User · Answer

It seems like the  in operator would serve your purposes just fine   You could do something like this  pseudo-query    if  db courses find   students       in     studentId     course    courseId    count    gt  0         student is enrolled in class     Alternatively  you could remove the  course    courseId clause and get back a set of all classes the student is enrolled in

User · Answer

I am trying to explain by putting problem statement and solution to it  I hope it will help  Problem Statement   Find all the published products  whose name like ABC Product or PQR Product  and price should be less than 15 -  Solution   Below are the conditions that need to be taken care of   Product price should be less than 15 Product name should be either ABC Product or PQR Product Product should be in published state    Below is the statement that applies above criterion to create query and fetch data    elements    collection- gt find               Array                   price    gt  Array    lt    gt  15                      or    gt  Array                               0   gt Array                                       product name   gt Array                                           in   gt Array                                               0    gt  ABC Product                                               1   gt  PQR Product                                                                                                                                                                                   state   gt Published

User · Answer

edit based on this now being possible in recent versions    Updated Answer  You can query the following way to get back the name of class and the student id only if they are already enrolled   db student find        id 0  name 1  students   elemMatch   eq ObjectId  51780f796ec4051a536015cf         and you will get back what you expected      name     CS 101    students      ObjectId  51780f796ec4051a536015cf          name     Literature       name     Physics    students      ObjectId  51780f796ec4051a536015cf          Original Answer  It s not possible to do what you want to do currently   This is unfortunate because you would be able to do this if the student was stored in the array as an object   In fact  I m a little surprised you are using just ObjectId   as that will always require you to look up the students if you want to display a list of students enrolled in a particular course  look up list of Id s first then look up names in the students collection - two queries instead of one    If you were storing  as an example  an Id and name in the course array like this               id    ObjectId  51780fb5c9c41825e3e21fc6             name     Physics            students                       id  ObjectId  51780f796ec4051a536015cf    name   John                     id  ObjectId  51780f796ec4051a536015d0    name   Sam                 Your query then would simply be   db course find                          students                           elemMatch                             id   ObjectId  51780f796ec4051a536015d0                              name    Sam                                                                            If that student was only enrolled in CS 101 you d get back      name     Literature       name     Physics           name     CS 101        students                             id    ObjectId  51780f796ec4051a536015cf                 name     John

[mongodb] MongoDB: How to find out if an array field contains an element?

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