How to filter array in subdocument with MongoDB

Question

I have array in subdocument like this          id    ObjectId  512e28984815cbfcb21646a7         list                             a    1                                   a    2                                   a    3                                   a    4                                   a    5                     Can I filter subdocument for a   3  My expect result below          id    ObjectId  512e28984815cbfcb21646a7         list                             a    4                                   a    5                     I try to use  elemMatch but returns the first matching element in the array  My query   db test find     id    ObjectId  512e28984815cbfcb21646a7             list              elemMatch                 a     gt 3                                The result return one element in array      id    ObjectId  512e28984815cbfcb21646a7     list         a    4         and I try to use aggregate with  match but not work  db test aggregate   match   id ObjectId  512e28984815cbfcb21646a7     list a    gte 5         It s return all element in array          id    ObjectId  512e28984815cbfcb21646a7         list                             a    1                                   a    2                                   a    3                                   a    4                                   a    5                     Can I filter element in array to get result as expect result

User · Answer

Above solution works best if multiple matching sub documents are required. $elemMatch also comes in very use if single matching sub document is required as output

db.test.find({list: {$elemMatch: {a: 1}}}, {'list.$': 1})

Result:

{
  "_id": ObjectId("..."),
  "list": [{a: 1}]
}

User · Answer

Using aggregate is the right approach  but you need to  unwind the list array before applying the  match so that you can filter individual elements and then use  group to put it back together      db test aggregate          match    id  ObjectId  512e28984815cbfcb21646a7             unwind    list           match    list a     gt  3            group    id     id   list    push    list a          outputs        result                   id   ObjectId  512e28984815cbfcb21646a7           list             4          5                       ok   1     MongoDB 3 2 Update  Starting with the 3 2 release  you can use the new  filter aggregation operator to do this more efficiently by only including the list elements you want during a  project   db test aggregate          match    id  ObjectId  512e28984815cbfcb21646a7             project            list    filter                input    list               as   item               cond    gt      item a   3

User · Answer

Use  filter aggregation     Selects a subset of the array to return based on the specified   condition  Returns an array with only those elements that match the   condition  The returned elements are in the original order    db test aggregate         match    list a     gt 3         lt -- match only the document which have a matching element       project            list    filter                input    list               as   list               cond    gt      list a   3      lt -- filter sub-array based on condition

[mongodb] How to filter array in subdocument with MongoDB

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