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Phil
Phil

Reputation: 7566

NoSQL/MongoDB: When do I need _id in nested objects?

I don't understand the purpose of the _id field when it refers to a simple nested object, eg it is not used as a root entity in a dedicated collection.

Let's say, we have a simple value-object like Money({currency: String, amount: Number }). Obviously, you are never going to store such a value object by itself in a dedicated collection. It will rather be attached to a Transaction or a Product, which will have their own collection.

But you still might query for specific currencies or amounts that are higher/lower than a given value. And you might programmatically (not on DB level) set the value of a transaction to the same as of a purchased product.

Would you need an _id in this case? What are the cases, where a nested object needs an _id?

Upvotes: 0

Views: 915

Answers (1)

Alex Blex
Alex Blex

Reputation: 37048

Assuming the _id you are asking about is an ObjectId, the only purpose of such field is to uniquely identify an object. Either a document in a collection or a subdocument in an array.

From the description of your money case it doesn't seem necessary to have such _id field for your value object. A valid use case might be an array of otherwise non-unique/non-identifiable subdocuments e.g. log entries, encrypted data, etc.

ODMs might add such field by default to have a unified way to address subdocuments regardless of application needs.

Upvotes: 2

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