MongoDB find() not getting anything
db.comments.find( { 'reply.recomm' : '13' })
db.comments.find( {'_id' :  ObjectId("5a6d9a1ee183f06dfd91dbf0"), 'reply.recomm' : '12' })


Both above query is looks fine and will be fetch the documents of records inside the comments collections. 

I am sure that you are doing mistake something like not in the tree of particular database  where your comments collection reside.

Let's from beginning see here,

First i have create the StackExchange database.

&gt;Use StackExchange


Then i have created the comments collection inside the StackExchange database.

&gt; use StackExchange
switched to db StackExchange
&gt;

&gt; db.createCollection("comments")
{ "ok" : 1 }


And inside of comments collections i have insert the documents, which you have provided in souce code.

&gt; db.comments.insert({
...     "_id" : ObjectId("5a6d9a1ee183f06dfd91dbf0"),
...     "date" : "20180126",
...     "oid" : "000",
...     "aid" : "0000000000",
...     "currentavalible" : "7",
...     "deletedbyuser" : "0",
...     "reply" : [
...         {
...             "oidaid" : "000000",
...             "nickname" : "****",
...             "date" : "2018-01-26 07:32:45",
...             "recomm" : "13",
...             "unrecomm" : "0",
...             "contents" : "aaaaaaaaaaaa"
...         },
...         {
...             "oidaid" : "000088",
...             "nickname" : "para****",
...             "date" : "2018-01-26 08:29:54",
...             "recomm" : "12",
...             "unrecomm" : "0",
...             "contents" : "bbbbbb"
...         }
...     ]
... })
WriteResult({ "nInserted" : 1 })


Now i want to fetch comments documents records from the comments collection, which is in StackExchange database.
First check through mongo shell, what is your current database like that

&gt; db
StackExchange
&gt;


Here it mongo shell the current database is StackExchange. 

Note:- I have create comments collection in StackExchange database. But for your confirmation , you must always checks before queering the query in mongo shell like



  show collections
  comments


Then try to find the documents inside the collection through the db.collection.find().


  Note:- Here collection is the collection name. In your case it's
  comments.


From the first query db.comments.find( { 'reply.recomm' : '13' })

&gt; db.comments.find( { 'reply.recomm' : '13' }).pretty()
{
        "_id" : ObjectId("5a6d9a1ee183f06dfd91dbf0"),
        "date" : "20180126",
        "oid" : "000",
        "aid" : "0000000000",
        "currentavalible" : "7",
        "deletedbyuser" : "0",
        "reply" : [
                {
                        "oidaid" : "000000",
                        "nickname" : "****",
                        "date" : "2018-01-26 07:32:45",
                        "recomm" : "13",
                        "unrecomm" : "0",
                        "contents" : "aaaaaaaaaaaa"
                },
                {
                        "oidaid" : "000088",
                        "nickname" : "para****",
                        "date" : "2018-01-26 08:29:54",
                        "recomm" : "12",
                        "unrecomm" : "0",
                        "contents" : "bbbbbb"
                }
        ]
}


and from the query
 db.comments.find( {'_id' :  ObjectId("5a6d9a1ee183f06dfd91dbf0"), 'reply.recomm' : '12' })

&gt; db.comments.find( {'_id' :  ObjectId("5a6d9a1ee183f06dfd91dbf0"), 'reply.recomm' : '12' }).pretty()
{
        "_id" : ObjectId("5a6d9a1ee183f06dfd91dbf0"),
        "date" : "20180126",
        "oid" : "000",
        "aid" : "0000000000",
        "currentavalible" : "7",
        "deletedbyuser" : "0",
        "reply" : [
                {
                        "oidaid" : "000000",
                        "nickname" : "****",
                        "date" : "2018-01-26 07:32:45",
                        "recomm" : "13",
                        "unrecomm" : "0",
                        "contents" : "aaaaaaaaaaaa"
                },
                {
                        "oidaid" : "000088",
                        "nickname" : "para****",
                        "date" : "2018-01-26 08:29:54",
                        "recomm" : "12",
                        "unrecomm" : "0",
                        "contents" : "bbbbbb"
                }
        ]
}
&gt;


But from the mention below both query you shall not fetch the documents records from the comments collection due to their improper fetching of Array records.

db.comments.find( { 'reply' :{ 'recomm' : '13'}  })

db.comments.find( {'_id' :  ObjectId("5a6d9a1ee183f06dfd91dbf0"), 'reply' : { 'recomm' : '12'} })


As per MongoDB 3.6 Documented Here . If you want to fetch the array documents then you can use $ (projection), Or you can use Aggregation Pipeline Stages.

As you said only you want to find the records in reply that  "recomm" is 13.
so, you have to use  $elemMatch (projection)operator and your query will be as mention below

&gt; db.comments.find({"_id" : ObjectId("5a6d9a1ee183f06dfd91dbf0")},{reply: {$elemMatch: {recomm: "13"}}}).pretty()
{
        "_id" : ObjectId("5a6d9a1ee183f06dfd91dbf0"),
        "reply" : [
                {
                        "oidaid" : "000000",
                        "nickname" : "****",
                        "date" : "2018-01-26 07:32:45",
                        "recomm" : "13",
                        "unrecomm" : "0",
                        "contents" : "aaaaaaaaaaaa"
                }
        ]
}
&gt;


After the OP requirement to edit the code and execute the query to find out OP desire results.

Here i want to insert the second documents which having different id field records in comments documents.

&gt; db.comments.insert({
... ...     "_id" : ObjectId("5a6d9a1ee183f06dfd91dbf1"),
... ...     "date" : "20180126",
... ...     "oid" : "000",
... ...     "aid" : "0000000000",
... ...     "currentavalible" : "7",
... ...     "deletedbyuser" : "0",
... ...     "reply" : [
... ...         {
... ...             "oidaid" : "000000",
... ...             "nickname" : "****",
... ...             "date" : "2018-01-26 07:32:45",
... ...             "recomm" : "13",
... ...             "unrecomm" : "0",
... ...             "contents" : "aaaaaaaaaaaa"
... ...         },
... ...         {
... ...             "oidaid" : "000088",
... ...             "nickname" : "para****",
... ...             "date" : "2018-01-26 08:29:54",
... ...             "recomm" : "12",
... ...             "unrecomm" : "0",
... ...             "contents" : "bbbbbb"
... ...         }
... ...     ]
... ... })
WriteResult({ "nInserted" : 1 })


Now i only find out the comments collection documents , which having the "recomm":"13" records.

&gt; db.comments.find({},{reply: {$elemMatch: {recomm: "13"}}}).pretty()
{
        "_id" : ObjectId("5a6d9a1ee183f06dfd91dbf0"),
        "reply" : [
                {
                        "oidaid" : "000000",
                        "nickname" : "****",
                        "date" : "2018-01-26 07:32:45",
                        "recomm" : "13",
                        "unrecomm" : "0",
                        "contents" : "aaaaaaaaaaaa"
                }
        ]
}
{
        "_id" : ObjectId("5a6d9a1ee183f06dfd91dbf1"),
        "reply" : [
                {
                        "oidaid" : "000000",
                        "nickname" : "****",
                        "date" : "2018-01-26 07:32:45",
                        "recomm" : "13",
                        "unrecomm" : "0",
                        "contents" : "aaaaaaaaaaaa"
                }
        ]
}