db.collection.find( { users: userID } ) vs $in operator for userItems

[William Simetin-Grenon]

I was wondering what would be the most efficient method to query a user’s items.

I could query for all items containing the user's id with db.collection.find( { users: userID } ) or store each item id of a user in the user itself and query them from that array with db.collection.find({ "_id": { "$in": [userItemsIDs] } }).

Is one method better than the other?

[Oleg Toropov]

Since MongoDB is document oriented DB, it is assumed that you keep all documents for user in one place (there is a limit on the size of the record - 16MB).

Simple, right? But the answer is more complicated.

1) Obviously if most of the queries will need all items than keep them all in one pace. It is easy to maintain the integrity of the record (update, delete)

2) If there are some items that will be used rarely in queries, or the items are  too big and cannot be kept in one record, then you _have_to_ design the schema to optimize your application.

I would recommend to take free training with MongoDB University 

https://www.mongodb.com/nosql-explained

https://university.mongodb.com/courses/catalog

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// Oleg V.Toropov, MCP | oleg.t...@gmail.com
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