MongoDB storing arrays of ObjectId's

Question

In my database I have to store an array of object ids. What should I use? Something like this:

[ObjectId("50350e12a36feb1be6000364"), ObjectId("57350e12a37fef1be6000922"), ObjectId("10350e17d34ffb1be6200925")]

or something like this:

["50350e12a36feb1be6000364", "57350e12a37fef1be6000922", "10350e17d34ffb1be6200925"]

I could save space with the second, and then cast to ObjectId, but am I loosing anything by using this approach? Do ObjectIds behave like foreign keys in relational databases?

Answer 1

I would definitely go with the first approach, storing the ObjectIds directly. This saves space, as ObjectId is 12 bytes whereas the second approach string is 24 bytes.

Also, if the ObjectIds are used to fetch the objects later, storing as ObjectId saves some hassle.

Answer 2

Unless you have a good reason not to, store them as an array of ObjectIds. It's more compact (12 bytes vs. 24) and it more accurately reflects what's stored. It can also enable driver-level support for following ObjectId references.