Get IDs of nodes via the edge collection only

Question

I am writing an application that stores external data in ArangoDB for further processing inside the application. Let's assume I am talking about Photos in Photosets here.

Due to the nature of used APIs, I need to fetch Photosets befor I can load Photos. In the Photosets API reply, there is a list of Photo IDs that I later use to fetch the Photos. So I created an edge collection called photosInSets and store the edges between Photosets and Photos, although the Photos are not there yet.

Later on, I need to get a list of all needed Photos to load them via the API. All IDs are numeric. At the moment, I use the following AQL query to fetch the IDs of all required Photos:

FOR edge
IN photosInSets
RETURN DISTINCT TO_NUMBER(
  SUBSTITUTE(edge._from, "photos/", "")
)

However... this does not look like a nice solution. I'd like to (at least) get rid of the string operation to remove the collection name. What's the nice way to do that?

Did the answer fullfill your needs? If not, whats missing? If, can you mark it accepted? — dothebart

David Thomas David Thomas · Accepted Answer · 2016-05-12T01:05:06

One way you can find this is with a join on the photosInSets edge collection back to the photos collection.

Try a query that looks like this:

FOR e IN photoInSets
LET item = (FOR v IN photos FILTER e._from == v._id RETURN v._key)
RETURN item

This joins the _from reference in photoInSets with the _id back in the photos collection, then pulls the _key from photos, which won't have the collection name as part of it.

Have a look at a photo item and you'll see there is _id, _key and _rev as system attributes. It's fine to use the _key value if you want a string, it's not necessary to implement your own unique id unless there is a burning reason why you can't expose _key.

With a little manipulation, you could even return an array of objects stating which photo._key is a member of which photoSet, you'll just have to have two LET commands and return both results. One looking at the Photo, one looking at the photoSet.

I'm not official ArangoDB support, but I'm interested if they have another way of doing this.

Get IDs of nodes via the edge collection only

1 Answers