My program generate 2 collection everyday in my database. Each day I have to compare these 2 collection and have to find out whether some specific elements of the documents are same in both the collection. If they are same I have to store the document somewhere.

Note: Each collection have 94.6k documents. Each document contains these columns or elements – _id(which is different all the time), studentName, timeOfEntry, timeOfExit, section, class.

*I also have one more collection named as names which. contains name of all students(which are unique) *

I have to find out which document in the 2 collection have same value for studentName, timeOfEntry and timeOfExit. How can I achieve this?

Example:

Like in collection1 -- a document contain the value of student:"Isha" ,entryTime:5pm and exitTime: 7pm.

collection 2 also contain a document with these values.

My approach: ** I was trying to create a function in springboot java where -- function(){ compare the 2 collection }**

CodePudding user response：

Query

• union the 2 collections
• group by the 3 fields, and count the occurrences
• keep only the duplicates (count>1)
• fix the structure of the document
• $out to write to the new collection

Test code here

coll1.aggregate(
[{"$unionWith":{"coll":"coll2"}},
 {"$group":
  {"_id":
   {"studentName":"$studentName",
    "timeOfEntry":"$timeOfEntry",
    "timeOfExit":"$timeOfExit"},
   "count":{"$sum":1},
   "doc":{"$first":"$$ROOT"}}},
 {"$match":{"$expr":{"$gt":["$count", 1]}}},
 {"$replaceRoot":{"newRoot":"$doc"}}
 {"$out": "newCollectionWithDuplicates"}])