What is the difference between save and insert in Mongo DB?

Let us consider the two cases here for save :-

1) Having _id in doc.

2) Not having _id in doc.

                        Save ()
                        /     \
                       /       \

                 Having _id     Not Having _id 

  ->In this case save will do    ->  It will do normal insertion 
    upsert to insert.Now             in this case as insert() do.
    what that means, it means 
    take the document and replace 
    the complete document having same
    _id.

Let us consider the two cases here for insert:-

1) Having _id of doc in collection.

2) Not having _id of doc in collection.

                        Insert()
                       /        \
                      /          \

   Doc Having _id in collection    Doc Not Having _id 
  ->  E11000 duplicate key     ->Insert a new doc inside the collection.
      error index:       

Save Vs Insert :

In your given examples, the behavior is essentially the same.

save behaves differently if it is passed with an "_id" parameter.

For save, If the document contains _id, it will upsert querying the collection on the _id field, If not, it will insert.

If a document does not exist with the specified _id value, the save() method performs an insert with the specified fields in the document.

If a document exists with the specified _id value, the save() method performs an update, replacing all field in the existing record with the fields from the document.

---

Save vs Update :

update modifies an existing document matched with your query params. If there is no such matching document, that's when upsert comes in picture.

• upsert : false : Nothing happens when no such document exist
• upsert : true : New doc gets created with contents equal to query params and update params

save : Doesn't allow any query-params. if _id exists and there is a matching doc with the same _id, it replaces it. When no _id specified/no matching document, it inserts the document as a new one.