Importing json from file into mongodb using mongoimport

I faced opposite problem today, my conclusion would be:

If you wish to insert array of JSON objects at once, where each array entry shall be treated as separate dtabase entry, you have two options of syntax:

1. Array of object with valid coma positions & --jsonArray flag obligatory

   [
     {obj1},
     {obj2},
     {obj3}
   ]
   

2. Use file with basically incorrect JSON formatting (i.e. missing , between JSON object instances & without --jsonArray flag

   {obj1}
   {obj2}
   {obj3}
   

If you wish to insert only an array (i.e. array as top-level citizen of your database) I think it's not possible and not valid, because mongoDB by definition supports documents as top-level objects which are mapped to JSON objects afterwards. In other words, you must wrap your array into JSON object as ALAN WARD pointed out.

Error:

$ ./mongoimport --db bookings --collection user --file user.json

2021-06-12T18:52:13.256+0530    connected to: localhost
2021-06-12T18:52:13.261+0530    Failed: error unmarshaling bytes on document #0: JSON decoder out of sync - data changing underfoot?
2021-06-12T18:52:13.261+0530    imported 0 documents

Solution: When your JSON data contain an array of objects then we need to use --jsonArray while import with the command like mentioned below

$ ./mongoimport --db bookings --collection user --file user.json --jsonArray
2021-06-12T18:53:44.164+0530    connected to: localhost
2021-06-12T18:53:44.532+0530    imported 414 documents

The mongoimport tool has an option:

--jsonArray treat input source as a JSON array

Or it is possible to import from file containing same data format as the result of db.collection.find() command. Here is example from university.mongodb.com courseware some content from grades.json:

{ "_id" : { "$oid" : "50906d7fa3c412bb040eb577" }, "student_id" : 0, "type" : "exam", "score" : 54.6535436362647 }
{ "_id" : { "$oid" : "50906d7fa3c412bb040eb578" }, "student_id" : 0, "type" : "quiz", "score" : 31.95004496742112 }
{ "_id" : { "$oid" : "50906d7fa3c412bb040eb579" }, "student_id" : 0,       "type" : "homework", "score" : 14.8504576811645 }

As you can see, no array used and no comma delimiters between documents either.

I discover, recently, that this complies with the JSON Lines text format.

Like one used in apache.spark.sql.DataFrameReader.json() method.

Perhaps the following reference from the MongoDB project blog could help you gain insight on how arrays work in Mongo:

https://blog.mlab.com/2013/04/thinking-about-arrays-in-mongodb/

I would frame your import otherwise, and either:

a) import the three different objects separately into the collection as you say, using the --jsonArray flag; or

b) encapsulate the complete array within a single object, for example in this way:

{
"mydata": 
    [
    {
          "project": "project_1",
          ...
          "priority": 7
    }
    ]
}

HTH.