AQL doesn't has the means to access collections dynamically, see: AQL:
function to get collection by name? (#1138)
<https://github.com/arangodb/arangodb/issues/1138>
You can look up documents dynamically however using DOCUMENT(handle).
This function has multiple signatures. For your use case, we can simply
provide a handle like "entry1/2" (collection "entry1", document with _key
"2").
FOR doc IN list
UPDATE doc WITH { data: DOCUMENT(doc.entry).data } IN list
The query will iterate over all documents in the collection "list", fetch
the content of the document with _id = doc.entry (e.g. "entry1/2"),
then extend the list document with an attribute key "data" and the
attribute value of "data" from the dynamically loaded document (entry
collections).
If there is no "entry" attribute, or the value doesn't refer to a document,
then a "data" attribute is still added, but with a *null *value.
This could be prevented like this:
FOR doc IN list
LET entry = DOCUMENT(doc.entry)
UPDATE doc WITH (
entry AND HAS(entry, "data")
? { data: entry.data }
: {}
) IN list
But there is actually a better solution:
FOR doc IN list
UPDATE doc WITH { data: DOCUMENT(doc.entry).data } IN list OPTIONS {
keepNull: false}
If the expression DOCUMENT(doc.entry).data returns *null*, it will be
ignored in the update thanks to keepNull = false.
This will not affect any other existing attributes with a *null *value,
only those updated (WITH clause) will be.
BTW: Instead of SPLIT(handle, "/")[0], you can use
PARSE_IDENTIFIER(handle).collection (or .key for the _key part).
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