CODEWITHSUNDEEP
mongodb
Gary Kipnis
Gary Kipnis

Reputation: 732

How to insert new document only if it doesn't already exist in MongoDB

I have a collection of users with the following schema:

{
_id:ObjectId("123...."),
name:"user_name",
field1:"field1 value",
field2:"field2 value",
etc...
}

The users are looked up by the user.name, which must be unique. When a new user is added, I first perform a search and if no such user is found, I add the new user document to the collection. The operations of searching for the user and adding a new user, if not found, are not atomic, so it's possible, when multiple application servers are connect to the DB server, for two add_user requests to be received at the same time with the same user name, resulting in no such user being found for both add_user requests, which in turn results with two documents having the same "user.name". In fact this happened (due to a bug on the client) with just a single app server running NodeJS and using Async library.

I was thinking of using findAndModify, but that doesn't work, since I'm not simply updating a field (that exists or doesn't exist) of a document that already exists and can use upsert, but want to insert a new document only if the search criteria fails. I can't make the query to be not equal to "user.name", since it will find other users.

Upvotes: 1

Views: 1341

Answers (1)

ZeMoon
ZeMoon

Reputation: 20274

First of all, you should maintain a unique index on the name field of the users collection. This can be specified in the schema if you are using Mongoose or by using the statement:

collection.ensureIndex('name', {unique: true}, callback);

This will make sure that the name field remains unique and will solve the problem of concurrent requests as you have specified in your question. You do not require searching when this index is set.

Upvotes: 1

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