What's the difference between insert(), insertOne(), and insertMany() method?

What's the difference between insert(), insertOne() and insertMany() methods on MongoDB

• db.collection.insert() as mentioned in the documentation inserts a document or documents into a collection and returns a WriteResult object for single inserts and a BulkWriteResult object for bulk inserts.

  > var d = db.collection.insert({"b": 3})
  > d
  WriteResult({ "nInserted" : 1 })
  > var d2 = db.collection.insert([{"b": 3}, {'c': 4}])
  > d2
  BulkWriteResult({
          "writeErrors" : [ ],
          "writeConcernErrors" : [ ],
          "nInserted" : 2,
          "nUpserted" : 0,
          "nMatched" : 0,
          "nModified" : 0,
          "nRemoved" : 0,
          "upserted" : [ ]
  })
  

• db.collection.insertOne() as mentioned in the documentation inserts a document into a collection and returns a document which look like this:

  > var document = db.collection.insertOne({"a": 3})
  > document
  {
          "acknowledged" : true,
          "insertedId" : ObjectId("571a218011a82a1d94c02333")
  }
  

• db.collection.insertMany() inserts multiple documents into a collection and returns a document that looks like this:

  > var res = db.collection.insertMany([{"b": 3}, {'c': 4}])
  > res
  {
          "acknowledged" : true,
          "insertedIds" : [
                  ObjectId("571a22a911a82a1d94c02337"),
                  ObjectId("571a22a911a82a1d94c02338")
          ]
  }
  

In what situation should I use each one?

The insert() method is deprecated in major driver so you should use the the .insertOne() method whenever you want to insert a single document into your collection and the .insertMany when you want to insert multiple documents into your collection. Of course this is not mentioned in the documentation but the fact is that nobody really writes an application in the shell. The same thing applies to updateOne, updateMany, deleteOne, deleteMany, findOneAndDelete, findOneAndUpdate and findOneAndReplace. See Write Operations Overview.

1. db.collection.insert():

   It allows you to insert One or more documents in the collection. Syntax:

   • Single insert: db.collection.insert({<document>});

   • Multiple insert:

     db.collection.insert([ , , ... ]);

   Returns a WriteResult object: WriteResult({ "nInserted" : 1 });

2. db.collection.insertOne():

   It allows you to insert exactly 1 document in the collection. Its syntax is the same as that of single insert in insert().

   Returns the following document:

   {
      "acknowledged" : true,
      "insertedId" : ObjectId("56fc40f9d735c28df206d078")
   }
   

3. db.collection.insertMany():

   It allows you to insert an array of documents in the collection. Syntax:

   db.collection.insertMany(
       { [ <document 1> , <document 2>, ... ] });
   

   Returns the following document:

   {
      "acknowledged" : true,
      "insertedIds" : [
         ObjectId("562a94d381cb9f1cd6eb0e1a"),
         ObjectId("562a94d381cb9f1cd6eb0e1b"),
         ObjectId("562a94d381cb9f1cd6eb0e1c")
      ]
   }
   

All three of these also allow you to define a custom writeConcern and also create a collection if it doesn't exist.

There is also a difference in error handling, check here. The insert command returns a document in both success and error cases. But the insertOne and insertMany commands throws exceptions. Exceptions are easier to handle in code, than evaluating the returned document to figure out errors. Probably the reason why they are deprecated in the drivers as mentioned in sstyvane's answer.