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

Question

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.

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 insert or update a document.

insert does only an insertion.

But in your case, it will do the same, as the document provided in save has no _id field.

By giving an example

Save an Apple

db.fruit.save({"name":"apple", "color":"red","shape":"round"})
WriteResult({ "nInserted" : 1 })

db.fruit.find();

{
    "_id" : ObjectId("53fa1809132c1f084b005cd0"),
    "color" : "red",
    "shape" : "round",
    "name" : "apple"
}

Save an apple with _id of previously saved apple

db.fruit.save(
{"_id" : ObjectId("53fa1809132c1f084b005cd0"),"name":"apple", 
"color":"real red","shape":"round"})

WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Now the apple we saved has, color updated from red to real red

db.fruit.find();
{
    "_id" : ObjectId("53fa1809132c1f084b005cd0"),
    "color" : "real red",
    "shape" : "round",
    "name" : "apple"
}

Save an apple with _id

db.fruit.save({"_id" : ObjectId("55551809132c1f084b005cd0"),
"name":"apple", "color":"real red","shape":"round"})

    WriteResult({ "nMatched" : 0, "nUpserted" : 1, 
"nModified" : 0, "_id": 55551809132c1f084b005cd0 })

Apple got inserted as there is no apple with the same Object Id to do an update

Insert an Orange

db.fruit.insert({"name":"orange", "color":"orange","shape":"round"})
WriteResult({ "nInserted" : 1 })

Orange is inserted

db.fruit.find();
{
    "_id" : ObjectId("53fa1809132c1f084b005cd0"),
    "color" : "real red",
    "shape" : "round",
    "name" : "apple"
}
{
    "_id" : ObjectId("53fa196d132c1f084b005cd7"),
    "color" : "orange",
    "shape" : "round",
    "name" : "orange"
}
{
    "_id" : ObjectId("55551809132c1f084b005cd0"),
    "color" : "real red",
    "shape" : "round",
    "name" : "apple"
}

So save will act as an update if supplied with an object id, provided the object id already exists other wise it does an insert.