Rest – Creating an entity relationship in REST: May I create the parent by posting to a child id

designdesign-patternsrest

We are currently designing a REST API to access classical customer data.
One of the elements in the API are the assets of an user. The assets are added under a given service. The backend API will only add an asset to an user under a given service. So, there's no User–Asset relation, but a User–[Service]–Asset relationship.

Our URI's will look like this:

/users/{id}/assets/{id}/services/{id}

Uses of the API will know the asset id and the service id to create a new entry.
What we are struggling with is the creation of this relation.

One straightforward way would be to post the whole relation to /users/{id}/assets/ 

POST /users/{id}/assets    
{asset:${id}, service:{id}, attribute1:"{var}", attribute2:"{var}"}

but then we are not actually creating an asset as the URI might indicate, but an asset-service relation.

As an alternative, we are considering POST'ing to the URI addressing the relation, like this:

POST /users/{id}/assets/{id}/service/{id}
{attribute1:"{var}", attribute2:"{var}"}

But in this case, the resource path /users/{id}/assets/{id} will not exist before the POST and will be created as a side-effect.

Is POST'ing to a resource path that does not exist yet allowed at all?

Thanks for your thoughts,

Gerard.

Best Answer

It sounds like you're suggesting that, whenever a user first posts to a non-existent relation, you will create it as part of the post.

If you're asking whether this kind of create-on-access pattern is a valid, acceptable development pattern, the answer is yes it is - it's both valid, and a fairly common pattern to see.

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