Database
db={Books:[{"BOOK_ID":"100","BOOK_NAME":"Book 1","BOOK_DESC":"abcd",},{"BOOK_ID":"101","BOOK_NAME":"Book 2","BOOK_DESC":"efgh",},{"BOOK_ID":"102","BOOK_NAME":"Book 3","BOOK_DESC":"ijkl",}],BookGroup:[{"GROUP_ID":"100","GROUP_NAME":"G1","GROUPS":[{"BOOK_ID":"100","BOOK_NAME":"Book 1"},{"BOOK_ID":"101","BOOK_NAME":"Book 2"}]},{"GROUP_ID":"101","GROUP_NAME":"G2","GROUPS":[{"BOOK_ID":"101","BOOK_NAME":"Book 2"}]},{"GROUP_ID":"102","GROUP_NAME":"G3","GROUPS":[{"BOOK_ID":"100","BOOK_NAME":"Book 1"},{"BOOK_ID":"102","BOOK_NAME":"Book 3"}]}]}
Query
db.BookGroup.aggregate([{"$unwind":"$GROUPS"},{"$match":{"GROUPS.BOOK_NAME":"Book 2"}},{"$group":{"_id":null,"group_name":{"$push":"$GROUP_NAME"}}},{"$project":{"_id":false,"group_name":true}}])