Question

i have a document:

"_id": ObjectId("THE DOCUMENT ID"),
   "GROUP NAME": "First Floor",
   "years": {
     "0": {
       "_id": "YEAR ID",
       "year_classes": {
         "0": {
           "class_name": "CLASS A",
           "_id": "CLASS ID",
           "participants": {
             "0": "PARTICIPANT ID" 
          } 
        },
         "1": {
           "class_name": "CLASS B",
           "_id": "CLASS ID",
           "participants": {
             "0": "PARTICIPANT ID" 
          } 
        },
      },
       "year_name": "YEAR 1" 
    },
     "1": {
       "_id": "YEAR ID",
       "year_classes": {
         "0": {
           "_id": "CLASS ID",
           "class_name": "CLASS C",
           "participants": [

          ] 
        } 
      },
       "year_name": "YEAR 2" 
    },
  } 

It is needed to add another class to a year. The code has the year id through the $YEAR_ID variable (is a string) which the class will be added to.

So, it has to check if the class name, given by $NEW_CLASS_NAME (string variable) has already been used in that year.

I saw the $and operator for mongo but i couldn’t manage to get it to work. If it were to be an SQL it would look likt: WHERE year_id=’$YEAR_ID’ AND class_name=’$NEW_CLASS_NAME’

How could this be done in MongoDB?

---

UPDATE:

The Query of:

{
“years.year_classes.class_name”: $NEW_CLASS_NAME,
“years._id”: $YEAR_ID
}

Is searching the document (so it will always return TRUE) not the same array.

Is there a way to direct the query to find those two values in the same array, so a class with that name, which is in the array of classes which is in the year array with that certain ID?

Answer 1

When you’re comparing two conditions inside an embedded object you need to use $elemMatch.
See the general explanation here.

Your query structure would be something like:

db.coll.find({"years":{$elemMatch:{"year_classes.class_name":$NEW_CLASS_NAME,"_id":$YEAR_ID }}})