What does "toArray() method loads into RAM all documents returned by the cursor" mean?

according to

Iterator Index

In mongosh, you can use the toArray() method to iterate the cursor and return the documents in an array, as in the following:

var myCursor = db.inventory.find( { type: 2 } );
var documentArray = myCursor.toArray();
var myDocument = documentArray[3];

The toArray() method loads into RAM all documents returned by the cursor; the toArray() method exhausts the cursor.

and toArray() , i can’t figure it out that cursors are pointing to disk space and when we call the batches by toArray method or other way , they collect info to ram? if not so if we have batches already in the cursor variable so we have documents in memory already! so what is the sentence " method loads into RAM all documents returned by the cursor" talking about

Welcome to the MongoDB Community Forums @Am_Ma!

The toArray() method in mongosh is a client-side method, so the reference to loading all documents into RAM is referring to fetching all matching documents from a query cursor into the local documentArray variable in your mongosh example.

For a large result set you will generally want to manually iterate the cursor and fetch smaller batches of documents for processing rather than retrieving the full result set.

For more information and examples, please see Iterate a Cursor in mongosh.


Tnx, Stennie for the answer.I do love mongodb, sadly it uses amazon infrastructure and it blocks my country ip addresses(iran).

i have another question which is why we use elementmatch while we have and? when i’ve studied mongo documents it has simillar usage of and
either using aggregate with match instead of using find method on collections

are these diffrences have diffrent performance too or they are just made for easier way to query writing?

Hi @Am_Ma,

In future, it is best to keep unrelated questions on separate topics so they can be addressed & solved independently :wink: .

These operators have similar outcomes for a simple equality match on an array field, but that’s actually a case where neither is required.

The $and operator performs a logical AND on an array of expressions. MongoDB queries use an implicit AND for a comma-separated list of expressions, so the typical usage of this operator is for a query that includes multiple expressions specifying the same field or same operator.

Without using $and, languages that represent query objects as hashes or dictionaries will merge duplicate keys. For example, in JavaScript:

>  myzoo = { 'a': 'Aardvark', 'a': 'Anteater' }
{ a: 'Anteater' }

The $elemMatch query operator is used when you want to match array elements that meet multiple criteria (i.e. at least one array element satisfies all the specified criteria). This is particularly useful for matching within arrays of embedded documents.


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